The regression occurred sometime within the past few days at most.
The “Discourse User Notes” plugin is enabled and I can view notes and add notes via the “User Notes” button on the user page without any problems.
Due to the requirement for staff privileges, I am not able to check for the fault on other Discourse forums, so I don’t know whether the fault is specific to the configuration of Arduino Forum or universal to any Discourse instance.
Actually I can reproduce this. I mis-read the second part of your post as opposite (not appearing in the user profile admin) I get the user notes option everywhere I expect except for in the post-admin options
I too have a hosted instance running the latest beta2-dev.
I suspect it may have to do with recent updates to the admin-post-menu.gjs file, and the plugin still using widgets: