Notifications stuck on mark all as read

[Penworks]

I can't report this as a bug because I don't know how it happened.

I run 2 Masto accts on moshidon android. One of the accts is fine, notifications work as expected in all scenarios. The other account notifications are 'stuck' on mark as read. No idea how this has happened. Though push notifications work and show new messages coming in, they don't show as unread in the app, and no number count shows in the bottom icon. There is an icon in top right of the notification interface that shows 2 ticks. Where did that come from? How is it turned off?

I'm sure I probably did something dumb, but no clue what and the other account doesn't have this problem.