I've recently replaced my old mesh WiFi routers [they died]; and the new ones  actually work "correctly" [maybe?] with IPv6 - at least I get IPv6 addresses and thinks mostly seen to be ok.

Mostly.  However, on my work provided Android phone, I've noticed that neither Outlook or Teams seem to get a working data connection - they don't refresh or update messages, and this evening when testing, I noticed I can't login on them either. 

If I turn off IPv6 at the router, and toggle the WiFi on the phone, Teams and Outlook  'just work'.

Seems you can't disable IPv6 on the phone; that's not supported. Pity; that would have been the easiest fix - I'd rather keep IPv6 functioning my other devices, including laptops, etc. 

Had anyone seen similar; or have any ideas on troubleshooting/resolution?

bng2-wlg1 if that's likely to have any bearing.