Yep, a work unit, of course.
In my case, I don't have the "Assign this tag to every new work unit" checkbox set for the tags in question, but these tags do have some tasks in the lists of "Assign this tag to new work units of these tasks:" option. So, yes, probably, this is the reason. And while you are there, could you please check why the tags get lost after the application is closed if these tags were duplicated and the duplicate was removed by hand? This is somehow related to the first issue, and maybe, it will get fixed automatically but maybe not, in which case, it would require separate attention.
