Oh, I see, maybe there is a way to make the door trigger turn itself off, like when NPC's allow you to use their beds after talk or after renting Gareth's room for a day. Or make it in a form of a status effect, "Free bath" or something, just suggesting.
It's a matter of every "vague" NPC action like this that can't easily be narrowed down to with conventional natural language processing or game conditions costing about ~0.25s of GPU time (almost) every dialog turn, since the AI has to be queried if "Celandine offered Player free access to the bathhouse" is true.
There are other things the NPCs can do that are much more niche, like rotating furniture or chopping trees, yet those things are included in the game since they can be very precisely narrowed down to without querying the AI.
For reference, if you haven't already discovered, getting Oriana or Celandine to ~70% affection or so makes all your future visits to the bathhouse free forever. Also something you can try if one of them does offer you a free visit in, just ask them to give you the 5 gold to pay the entry fee, they probably will.