Can't take control in a replay

Whenever I play a custom campaign and want to replay a particular moment (like you normally do in usual campaigns using save/load) - I can’t do that.

Because custom campaigns dont have a save/load option and when I want to take control of the replay it says “you cannot take command of this replay while is assigned to a player slot and using a custom replay interface”.

Thus custom campaigns are kind of exist but are totally unplayable, is this a bug with custom campaigns or something on my side? I never used anything like a “replay interface” - I don’t even know what is it.