I've debated for a long time as to whether or not I should share this observation as it is anecdotal.
My wife and I switched to Android phones last year, but keep our old iPhone 6's, without sim cards, for other purposes. I never bothered to remove the mobile devices from HE but did take them out of all automations. She plays games on her's, and I display a Wyze Cam when I'm at my desk. Apps are still updated (including the HE mobile app), but iOS is at 12.4.5.
Periodically, I like to look at my router's logs to see what or who has been trying to connect to my network, but wading through all of the re-connect entries(every 15 mins +/- from my wife's iPhone when asleep) was annoying.
Why did her iPhone do that and mine didn't? The only difference between them is her's was on the 5G band and mine was on the 2.4.
Interestingly, when I looked at the event logs for the HE mobile devices, my iPhone had been present for about two months, her's had been toggling dozens of times a day. We connected her phone to the 2.4 band and 'Forgot' the 5G band in the wifi settings and to my amazement, the wifi disconnects/reconnects stopped and her HE mobile device has remained present for several weeks.
As I said, this is anecdotal. I'm curious if someone who's having issues with HE mobile app presence, an up-to-date iPhone with up-to-date firmware and a sim card, were to limit their phone to only 2.4 wifi, would they see a difference in behavior?