Well… I solved the problem myself. I have no clue how this could have been ticked but I gave myself the idea bringing up Cvars.
I have the addon AdvancedInterfaceOptions, which has a Cvar browser. For me the issue was the “showTamers” Cvar was set to 0… Usually the addon also would tell me when another addon last toggled a Cvar, alas it didn’t show it here. Which implies I did it myself somehow but I doubt that.
Anyway, if you’re stumbling upon this thread in the future with the same problem, here’s the in-game command for this Cvar:
/console showTamers 1