Thank you very much for your diligent look into this and the great explanation. That explains my observations.

Sadly, it seems this is still happening. It just happened twice at [[w:en:User_talk:Eteethan#Why_remove_tag.2C_instead_of_using_a_template.3F]].

While it is of course true that "article exists as a redirect != article does not exist", it does not logically follow that this request is "invalid". In fact, this request addresses a real existing problem, which is, as @Alsee writes, "confusing to the point of upsetting" and therefore merits consideration.

I started a separate room for discussion of @Izno's alternate solution: Z276.

From Izno's message above it seems that we all agree that it's an issue, but we may not agree on the specific solution. So it may be appropriate to change the title simply to "Bonnie and Clyde problem".

I am setting the priority to "Normal" because, as @Tgr said, with many echoing the sentiment:
"It breaks one of the most fundamental functions of the wiki software, that a reader can easily find more information on the topic in a different language. With Wikidata, subjective editorial decisions about article scope [...] break the navigation between languages. [...]".

This has high priority because mention notification is being used by many people, and if one can not rely on it, it becomes utterly useless (or worse than that for those people who do rely on it). I just experienced this problem for the third time at [[:en:User_talk:Chris_the_speller#The_page_.22Resident_Magistrate.22_has_been_moved_to_.22Resident_magistrate.22]].

Since this has been "floating" here since August, I think it may be helpful as a motivation to translate the gist of the German discussion:

I am afraid this request is a waste of time, and can even backfire.