Hi Graeme,
personally, I've not observed this issue, in my phone (a Samsung Galaxy A50 with Android 11), OsmAnd has received several updates during the past 2 months and the maps were never been deleted.
Can you please check what is the location you have set for map storage, by looking at menu Settings -> OsmAnd settings -> Data storage folder ?
I have "External memory 1", that corresponds to /storage/emulated/0/Android/data/net.osmand.plus/files.
I also have installed OSmAnd+ (v.4.0.9) in a MEmu virtual machine, that emulates a OnePlus device with Android 7.1.2: the setting there is the same as above, and again the app updated suceesfully without any issue.
Cheers,
Max