This problem was noticed first at German Wikipedia, with a permalink inbolving a ö. I ran some test at French Wikipedia, as French language has diacritics too.
I took a pair of topics on https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Le_Bistro/25_janvier_2024. These messages were transcluded to https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Le_Bistro earlier this week. I emulated the case of a user who accessed and got access to the message when they were on the main page.
- Wikipédia:Le_Bistro#Informations_à_la_création_de_compte returns a "not found" message
- Wikipédia:Le_Bistro#Vote_pour_suppression_de_bandeaux gets a notifucation with links to the right locations.
According to @matmarex:
ö is double-encoded in the API request. Probably easily fixable
I'm looking for other cases, and I'm sure any language with diacritics will observe this quite serious issue.