Predictably, while I was trying to verify that things work right when adding the relevant permissions back into a new user one by one, I ran into yet another new problem. After some investigation, I filed it as https://folio-org.atlassian.net/browse/DMOD-240#icft=DMOD-240.
The permission n mod-users have all changed – see https://folio-org.atlassian.net/browse/DMOD-180#icft=DMOD-180 – so now the Users UI app is hiding things it should be displaying, such as the viewed user's permissions.
So these will need changing. *sigh*