I wonder if just for grins you take ONE out of the miscreant group and totally remove it from HE, then reintroduce it like it never was apart of the system. Then just keep an eye on any differences in behavior.
Can't wait to hear the end of this story. I have a number of these that are serving "mission critical" duty and I would hate to have them start dropping out.
I have these and all kinds of environments and I've never had any issues like you're talking about, except when the device was starting to fail. Unfortunately.
One thing you could try just to confirm it isn't related to a problem with moisture is remove one from one of the bathrooms so it's not involved in any humidity and see if anything changes.
Maybe swap the one in the garage with one of the ones in the bathrooms is one of the best ways of confirming it's not just a bathroom issue.
I can try that, sure. But I have three in the bathroom including two that are literally right above the shower stall, and those two aren't affected.
I guess the device was starting to fail, I could probably bite the bullet and buy a new one except, the page for these on ebay is no longer there... maybe alex_sari sold them all? I know they are no longer manufactured, or so I'm told.
They do show up from various sellers from time to time, and IMHO are worth hunting for. I've had a couple go south, but so far only ones that I've manage to drop onto tile/cement floors. My clumsy genes are very powerful. 
Good luck on finding more. Iris v2 or v3 are both great, I actually like the v2 better as I don't generally use the humidity reporting added to the v3, and like the v2's smaller size.
Dumb question, but have you tried replacing the battery? I know on my one v3 that when it gets low it can get squirrely. (Never rely on what a device tells you the battery level is. 9 times out of 10 it's lying like a politician)
You can also hard wire the motion sensors if you have an outlet nearby or run a low voltage wire in the wall bay to a transformer.
There are other threads on this as well..
I've had one or two Iris V2 motion sensors that stopped reliably reporting motion.
The temperature reporting of these sensors continued to function correctly so I now use them in my fridge/freezers to monitor the temperature.
Repurpose for the win!!! That's a nice idea thanks.
My V2's are still humming along 
- the only issue I had was a starting to fail USB plug for my wired (see above) sensor. After replacing the USB plug all good.
Was just thinking about how easy it is to deploy sensors... but that dealing with the battery aspect and any power related "device in decline" syndrome within a growing population of sensors.... is just a hassle.
Off Topic:
The "Device of the Year" Awards ought to have a corollary award -
Device of the Years , a badge of honor earned over time;
that being a list of those devices that never cause trouble,
rarely need a new battery, and just work;
extra points for working better than what their price tag would warrant.
If I was a vendor in this space my goal would be to have my brand recognized for that,
being "first to market" with new fangled gizmos would be secondary because sure enough if they aren't reliable it doesn't matter how fancy they are.
whoa, i never thought of that. They work ok in fridges? I could probably put one in there and have it send an alert when the temp gets below a certain level.
No, I bought all my V3's at the same time and all are running at 100% with the batteries that came with them. Next time it malfunctions, I'll consider trying that, though. At the risk of jinxing it, the two problem sensors have actually behaved for the last two days.
Yep. And freezers too. I have the bad motion sensor in my deep freezer (subject to -15F to -20F and it's been shivering along for some time now. I also have a couple of old Iris V1 contact sensors in my house fridge/freezer.
I know they work as we get notifications when my bride reorganizes and the temps climb.
I also added contact sensors on the freezer doors to let me know if they get left open. They have saved us a couple times. We lost an entire freezer once when the door was cracked open and everything defrosted.
This!
Definitely a life saver.
Yeah I have contact sensors on the doors of our basement fridge. I have an alert that gets broadcast via our sonos when either door (freezer/fridge) is open for too long.. has worked great.
The internal sensor seems like a nice idea as well in case door sensor is out of batteries or some mechanical issue...
Awww...I was just choosing my favorite popscicle. Scared the bejeesers out of me!!
Yep, daughter did a late night snack run and left the door open. That was a fun few minutes trying to figure out what the weird noise coming from our kitchen was (Star Trek hailing alert followed by a muffled voice).
I have 4 iris sensors that I recently migrated over from SmartThings. They worked nearly flawlessly in ST and with Hubitat, I can't hardly get 1 day of use out of them before they quit working. I typically have to do a battery pull or repair them with the hub. Super frustrated because I know its not the sensors that have the issue.
How many mains powered Zigbee repeaters do you have? I have over 20 Iris sensors and they all work great but I also have a string Zigbee mesh with 30 mains powered switches.
Probably just a couple. Thing is I never had problems with them whatsoever in their current locations until I switched to Hubitat.
You definitely sound as if you have a weak mesh.... I would throw a couple of repeaters in there (the sengled oval outlets make great ones)
https://docs2.hubitat.com/en/how-to/build-a-solid-zigbee-mesh
Download the Hubitat app
