I’ve just successfully tested it on the JP too, while my X10iii is still on 5.1.0.10 (the minor update should be unrelated to this).
On the latter I have microG at version 0.3.15.2503932
Can you check in microG Settings to see if anything that might trigger a rejection needs sorting.
I have most of the Self Check settings checked.
I also have Google device registration. Cloud Messaging, and Allow device attestation (though Natwest doesn’t seem to use it) enabled.
Under Play Store services I have all the options enabled.