Can't edit payments on a shared tab

I added a custom payment to a shared tab between me and my girlfriend and now I want to remove it from the tab. When I click ‘EDIT’ at the top right, nothing happens. It used to reveal minus signs that would let you delete payments from the tab. This happens on my girlfriends phone too. Clicking the ‘EDIT’ button does nothing. The only other significant thing I might mention is that the tab is 2-3 years old and we use it on a regular basis so it probably has a lot of data in it.

Whilst on a shared tab, click on ‘EDIT’. But it does nothing.
Android 11
Google Pixel 5
Monzo 3.71.1
(The above details are correct for myself and my girlfriends phone)