Hi, the issue was that 10.5 happily accepted any button combo as a closedown emulator option, but it simply didn't work in practice. Could exit emulator via Esc key and whatever specific exit function were available within, but not from the controller (the controller otherwise worked perfectly).
I mostly fixed it by restoring a backup of 10.1, and then upgrading again. Hence, I traced the problem to the auto-upgrade failing the first time I tried it, which messed up some settings or something like that. Regret I can't replicate.
One quirk...the use of Start+Select as an Exit combo now works in Retroarch, it works in standalone emus (Altirra is my favourite, I have a few others), but oddly doesn't work in standalone MAME64 (as opposed to MAME-core-run-thru-retroarch). Not sure if there's anything different I have to do setup-wise (it's always been a fiddly emu to work with). I only really use it for Bally Astrocade and a few obscure ex-MESS systems, but curious to establish what's up with this one.