If she is only urinating in the bathtub while bathing and not anywhere else, it is most likely caused by the warm bath water. I always have my son use the potty before he takes a bath because if he doesn't, the warmth stimulates a reaction that causes him to urinate in the bathtub.

Even, I myself, feel the need to urinate with warm water. So I too will potty before I get into the bathtub or shower. My grandma used to use warm baths for me, because I have always had constipation issues. So she'd make me sit in a very warm bath for awhile until I needed to potty. It worked.

However, if you're concerned about it being a medical issue (I've never heard of tethering, I'll need to look that up!), then I'd reach out to a doctor. Just to be on the safe side, considering your concerns aside from the bathtime urination.