This is technically necessary because otherwise the Mediawiki thinks this is a user page of a non-existent user. Subpages must always be separated with a "/".
Sujet sur Discussion utilisateur:TomT0m
Oh OK, I got confused because that broke the pages that uses the template (listeria generated list, so they need to be updated before the renaming works) Used to gather data from Wikidata to load into a Wikipedia module, so It’s an unusual workflow. I don’t remember why I used ":" in the naming, maybe a typo.