Battery drain on 2018 CX-5 due to ODBII device

2018 CX-5 GT
The end of Nov 2021, my trusty 2014 Mazda6 got rear-ended and was deemed a total loss by my insurance company, Metromile. They are one of a select few companies that charge based on usage (i.e. mileage), using a small tracking device that ordinarily plugs into the ODBII port on the car. It worked fine on my Mazda6, but it was not to be on the CPO 2018 CX-5 GT that I purchased as my replacement. Several weeks after owning the CX-5 (mid-December, as it got colder), my battery needed a jumpstart, so I took it back to my Mazda dealer and they replaced the 3+ year original battery with a new one. A week or two later, the new battery needed a jumpstart, and so the adventure began. Because the dealer had no loaner cars to give me while they would diagnose and fixed the issue, I took daily voltage and temperature readings, while using the car sporadically, sometimes letting it sit for 2-3 days. On those occasions, the battery voltage would drop below 3.5v and would need a jumpstart. Finally, off to the dealer it went on a flatbed tow truck. They went straight to replacing the power liftgate module, which had a battery drain issue on the 2019/2020 CX-5s, but also removed the Metromile device from the ODBII port. After a week, they informed me that the car was starting fine, and that it was the Metromile device causing the problem. I pushed back, since nothing official in the Mazda documentation warned against such a device, and Metromile bases their entire business on it. Metromile did however have an alternative approach which is an adapter that plugs into the cigarette lighter (in the dashboard) which powers the device only when the car is running. The dealer agreed to plug the device back into my CX-5 over the weekend, and sure enough, the battery was dead after a cold weekend. The service manager insisted that nothing should be plugged into the ODBII port on these newer cars while the engine is not running, because the processors in the various subsystems (e.g. ECM) will not go into "sleep" mode while an active device (e.g. diagnostic scanner) is plugged in. While the device itself draws as low as 10 mA, the car processors, while awake, can draw as much as 500 mA, which will progressively, and more quickly, drain the battery, and much quicker as the temperature drops. All of this could have been avoided if Mazda had put a simple printed tape across the ODBII port warning not to plug into the port when the car if not running. Since switching to the alternative Metromile adapter, I have left my car for 7-10 days and it started, as it should, without a hitch.

I hope my weeks of grief and frustration documented here will help anyone struggling with the same puzzlement.
Huh, how bout that. I leave my obii bluetooth scanner plugged in all the time. Never an issue (19 signature 2.2 diesel)
Good to know.
I used to have a ScanGaugeII-X plugged into the OBDII port and left it there on my '08 CX9 (sold). Never had an issue.

Could be the newer system as you said.
To clarify a bit more...the newer car models have more electronic subsystems controlled by processors, some of which remain on for up to 20-30 minutes after shutting off the engine, and must go into sleep mode to avoid excessive battery drain. Also, ODBII devices are not all alike. Apparently, some have their own sleep modes that gracefully allow the car's processors to go to sleep. Unfortunately, Metromile's device is not one of them.
I keep a Scangauge plugged in and while the car's plugged into a battery maintainer it seems to cause the Scangauge to light up for a few seconds every once in a while. The readings say what they said when the engine was shut off. It's pretty weird.
I can concur with the OP and have had the same experience. I used to own a 2010 Mazda 3 and because it had no temperature gauge, I bought and installed an Autel Maxitrip TP100. It plugs into the OBDII port and it displayed selectable engine data. I left the TP100 plugged in for several years and never had an issue. In 2018 I bought a new CX-5 GT and this winter when the car wasn't heating up enough I plugged the TP100 into the 2018 CX-5 OBDII port to get a digital temperature readout.
I left it plugged in for 2 days over the weekend without starting the car and sure enough on Monday morning the battery was dead for the first time since new.
I would agree with the service manager who said that nothing should be left plugged into the OBDII port from my experience.
I bought an OBD II scanner [vGate vLinker MC+ - bluetooth] & recently started leaving it plugged in to my 2021 CX-5 SIG [car is started most days, maybe 3 days between at most] - had no problems. After a 200 mile drive, car was left outside un-started for 5 days [with said MC+ in the OBD II port] - when I then tried to open, car was as dead as a dodo so I had to use emergency key from the fob. Everything dead [including when using the buttons on electric seat]
Called AAA & the [not very talkative] mechanic sent tested my car battery with his gizmo [was like a 1.5" screen that showed what looked like a QR code that his phone then scanned and showed the info?] - it showed car battery was 2V [two volts - he was very surprised as he hadn't seen such a low voltage before on any car]. He also ran a battery test with his gizmo that passed
After he started car with his portable battery everything seems ok - the only thing reset in the car was the driver seat memory
I'm assuming it was the OBD scanner that flattened battery, so from now will only plug it in when I'm using it. MC+ claims "Operating Current is 24 mA, automatic sleep in idle state, sleep current is as low as 3mA" but, as noted above by others, it could be keeping the car ECU active
I subsequently bought a (commissions earned) that I haven't had a chance to use yet - probably OTT but I can also use it to charge one of my laptops [by USB-C PD] if push came to shove