@mangeurdenuage I always add with the remote subscription button that's on !loadaverage, but now I tried it on the other instance, and it didn't work indeed.
Weirdly, I can tell that shitposter.club did poke me with webfinger, yet "http://ostatus.org/schema/1.0/subscribe" was somehow beyond its reach.
Could be a bug in Pleroma, but just from looking at https://git.pleroma.social/pleroma/pleroma/-/blob/develop/lib/pleroma/web/web_finger/web_finger.ex I cannot really tell.