Ah ah ah - interesting, it appeared only after I moved away from the note and then back on it.
This is an automatically generated tag (part of the processing I am working on to turn a #hashtag in the text into an actual tag) and while the tag was added (I saw it when clicking on the tag icon) it was not visible.
Which means that the tags are refreshed when a note is parsed on opening, something I must be aware of. I was under the impression that if I have a note opened, it is refreshed periodically - which means that if I change a note in one place and I am synchronized to, say, Nextcloud - then the change will be reflected in the first note.
If this indeed the case, it may be worthwhile to also redraw (update in the app) the metadata (such as displayed tags)