I'd like to inquire about the status as well, though it seems it got better?
If I run Dropbox (v70.4.93) with IPv6-only (IPv4 completely disabled!) the client will connect briefly, see that everything is in sync, and then change to grey icon mourning about missing connectivity.
If I enable IPv4 on the interface, but don't provide a valid route (i.e. no IPv4 connections are possible) Dropbox seems to run okay and stay connected, except for some intermittent disconnects.
Please note: I don't have or run any NAT64. This is pure IPv6-only.