I was diagnosed recently as an adult but I do remember lying constantly as a child, for no particular gain. Interestingly, as an adult, I find that I pathologically tell the truth, even when the consequences of doing so will cause me personal detriment.
Personally, I wouldn’t make a big thing of it. I would think that your daughter thought she was going to get in trouble and so it’s a very logical thing to do. There are no major consequences to peeing in the bath, urine being 91-96% water diluted into about 100 litres and you’re talking minute amounts of urea and other salts. (I’m pretty sure each of my older brothers and sisters pee’d in the bath before it was my turn as a kids…).
As mentioned previously, I definitely wouldn’t take it personally or see it as a moral failing. Basically, don’t sweat the small stuff. All kids, ADHD or not, will lie to avoid getting in trouble.