I tried earlier to leave a dead realm (for more than a year) that I had in my list. Though it failed at leaving it, it didn’t fail a leaving a realm, as is, my active other one…

As I had only two realms in my list, I can’t say if it is random or what, though it could possibly be linked to REALMS-1692, as iirc there were no create your realm button at this time (and I tried to remove the last realm in my list, which was occupying the first —in the list— realm placeholder, so to speak).