I found the Support case you recently opened on this. I can see these APs are actually rebooting many times a day leading to the red patches you see on the timeline. Also, they're not actually running 28.5. For some reason they're still running a much much older firmware release.The firmware update is failing which is causing the constant upgrade attempt and reboots.
My advice is lean on Support to ensure these get the proper firmware and that should stabilize them. Also, 28.6 just hit RC status today so I would actually recommend using that version.
Yeah, I can see it's trying to upgrade, failing, rebooting over and over. I believe Support has the power to force an upgrade through. I've seen this happen a few times over the years although never on APs.
I've seen this problem on MX several times, and most of them can upgrade firmware properly after manual rebooting. Plus, in that case, I can see firmware upgrading message on Local status page. But I cannot see any of these messages on MR.
Update : I got answer from TAC, and they said that some kind of SSL inspection on on-premise side is rejecting MR's SSL certificate because it's "unknown CA". And it seems recently updated firmwares(MX/MS/MR) are all using SSL certificate.