I have two Mikrotik routers, a main one and another one that is set in repeater mode. The local IP range is set to 192.168.1.x (not the default 192.168.88.x). When a laptop is started within the physical range of the repeater (i.e. much closer to the repeater than the main router, so that it connects to the repeater), it gets a correct IP address. When a device moves from being connected to the repeater to being connected to the main router, the IP address stays the same (being in the correct range). When a device is started and connected to the main router, it also gets correct IP address.

However, if a device that has been connected to the main router moves physically much closer to the extender and gets connected to the repeater, sometimes it gets an 192.168.88.x IP address. This seems to have started after upgrading the two routers to RouterOS 7.1.5. My assumption is that there are some .88 settings left somewhere on the repeater, but I couldn't find any. Any idea what may be causing this?

Best to post config of both devices.
/export file=anynameyouwish
Review both configs for any leftovers of sensitive info.
Then post both separate between Code quotes.

Most likely while doing that review, you will see where that subnet is still being used.

To add to post by @holvoetn: serms like wifi repeater has DHCP server running. You should disable it if wifi repeater is really setup to be L2 transparent ....

@holvoetn and @mkx: thanks for your help. There was no DHCP server set on the repeater, but somehow the 88 range was in the Addresses tab under IP. It could be an artifact of a recent upgrade or it may have been there until that upgrade but not used. I don't remember whether that repeater was set from scratch or from the default settings, if from the default settings, this could explain why the 88 range was left there, but it doesn't explain why there was no issue for couple of years since the repeater was put in use.

Either way, I removed the 88 range from there and since then there were no 88 addresses assigned. However since upgrading to version 7.2, machines in the physical range of the repeater are sometimes getting 169.254.x.x addresses, from what I understand this means that they cannot reach the DHCP server on the main router. If after several minutes I am to force getting an IP address, they get correct IP addresses. My thinking is to set the repeater again after removing any settings currently on it and see what would happen.

Best to post config of both devices.
/export file=anynameyouwish
Review both configs for any leftovers of sensitive info.
Then post both separate between Code quotes.

@lap
Why hasn't you done it yet?
If you had done, the problem was probably already solved without wasting time in questions and guesses...