It doesn't matter what it's a symptom of though Monday,
The vast majority of people on here feel it's inappropriate or becoming inappropriate and it should be stopped. A small minority feels it's fine.
This does likely translate into what our wider society would think also, the majority would likely think this inappropriate or becoming inappropriate and should be stopped . Either side hurling insults at the other isn't going to change anyone's opinion.
And the op needs to take that on board. That the majority of people if they found out would likely feel this is inappropriate to varying degrees, from she's to old now but it's sweet, through to the dire sexual connotations being put forward.
Teachers may question it if it comes to their attention, parents will discuss it, the girl herself may get bullied by other kids about taking baths with her dad if her friends etc found out. The vast majority of her peer group will not be doing similar. And people and kids can be cruel. Teachers can be jumpy.
Personally if it was my kid, I'd recommend the husband finds another way to spend one on one time with the daughter, one that doesn't involve full nudity for them both, to replace the bath times, and then he changes his bathing habits for a short period until it's no longer an issue.
This child wants the time alone with her dad. She wants the attention. It is not about the bath, That's simply what she has to do to get that time. If she gets the exact same attention and one on one time with him in a different context, then there is no need for anyone to be sad.