Please forgive replying to my own post, but I think I've worked out why this bug has appeared, but I've no idea how to fix it.
I grabbed the working Serbia_europe.obf file from my phone, and ran:
./inspector.sh -vpoi ../Serbia_europe.obf | grep -i drinking | head
there's lots of entries, but they all look like:
sustenance: drinking_water 33 Lat 42.178764 Lon 21.27775 osmid=
4432989592sustenance: drinking_water 61 Lat 42.176937 Lon 21.28011 osmid=
4432989595
I grabbed the Austria_tyrol_europe_2.obf from osmand webserver, and ran:
./inspector.sh -vpoi ../Austria_tyrol_europe_2.obf | grep -i drinking | head
again, lots of entries, but these ones look like this:
man_made: drinking_water Lat 47.40821 Lon 10.747333 osmid=454561186
man_made: drinking_water Lat 47.119236 Lon 11.679368 osmid=1361126744
Picking up my phone, doing a POI overlay of "man made drinking water" and bingo, we have working POI, tho the icon is just an orange circle, not a picture of a tap like it is with drinking water POI.
This leaves the question: Is this a bug with the maps i.e. they should be "sustenance: drinking_water" as the app expects, or is this a bug with the app, which is expecting "sustenance: drinking_water", but should expect "man_made: drinking_water" ?
J