I found this on Fedora with Gramps 5.2, but it needs testing in other (current) versions and OSes.
The delete operation in the Citation Tree view mode of Sources category does not actually complete deleting a citation. Delete only completes when the row(s) is(are) a Source(s).
It starts with a freshly imported example.gramps tree.
Then in the Sources category, select the Citation Tree view mode and expand the “World of the Wierd” source.
Select either of the 2 citation rows and click the delete toolbar button or choose Edit → Delete.
There are 2 deletions confirmations with slightly different titles, the first has the Category in addition to the GrampsID. One with Cancel/Delete buttons, a second with Cancel/No/Yes buttons.
However, after confirming both, the deletion of the Citation is not performed.
(Note that when the selected row is a Source, deleting similarly has the 2 confirmations but the deletion of the Source is actually performed. And all the Citations for that source are also deleted. But there was a delay in the view being refreshed after the deletion before the view was refreshed.)
The behavior of Delete in the Sources category warrants some discussion. Since deleting a source also deletes every citation under that selected Source, it seems like that needs at least as much extra confirmation feedback as a multiple object deletion. Perhaps the second dialog should report a count of the sources and citations being deleted?