OpenHABian changing IP address

Manual editing of config is not supported in openHABian, only what’s accessible through openhabian-config.

And it’s so quick and easy to install to a fresh SD and import your config in one go that I don’t understand why you’re unwilling to attempt that.
You would be seeing within minutes if the reinstall (or side-install, it rather is) fixes your issue.
Then you can still decide whether you find the effort of reinstalling stuff worth it.
Backup tools you don’t need any extra, all you need to B&R config and OS incl 3rd party additions you made already is available as part of openHABian.

Sure, sounds like you have done all the easy stuff at this point only question I have is around this statement.

Pound signs in front of a item in a config file tell the OS to ignore that setting so if you entered this exactly as you show in your post then the test is irrelevant as you never actually used the static assignment you edited in.

none of your point a) should come into play here actually. And I would not expect you to have to redo it if you have not changed it recently.

Although most of the issues with ASUS refer to ISP assignments if you actually look on the ASUS community blog they talk quite a bit about the asus router having issues with its dhcp server assignments and lease times. Again I would make sure you have valid date and times for both your router (asus normally runs a NTP client that retrieves and sets your routers date and time from a web source. target clients like Rpi do not have a RTC so they handle it via a service openhabian does it that way also I sort of recall as well.

Manual assigned at client or you have associated a MAC address with a ip and you are using DHCP static renewal (That makes a difference as to the usefulness of that statement here) What do you have the lease renewal time set for on the router?
As much as I dislike the rebuild the OS as a troubleshooting step @mstormi is correct the next easiest step is to pop fresh SD card in and quickly spin up a virgin install and see if your issue reoccurs with all default settings.
As for any other next steps short of firing up wire shark and sniffing the traffic during the time this occurs i cannot think of another easy test to try.

Ok I did the full reinstall and actually it solved the issue. Unfortunately it’s not only copying the config, its also the mosquitto setup, samba, my motion videocam, some scripts, git, etc. but you’re right I tested it with a basic install, saw that the IP change didn’t occur within 4 hours and therefore was safe to continue with the “migration”. Thanks to all for your help, still it is a mystery to me what happened with my setup.

Probably your OH upgrade caused some Linux networking package to be updated.
What’s your OS (cat /etc/debian_version) and when approximately did you set it up ?
What’s your dhcpcd version ? (dpkg -l|grep dhcpc)
what exactly did you do to upgrade OH ? Some apt command ?