Sadly the potty mouth talking toilet has behaved like this from the launch of City Living, not a new bug at all. Sims have always interrupted baths with soaks or pretty much anything else they have been directed to do, to go look 'in awe' at the toilet. It's such a clumsy piece of animating.
You can always build a small separate toilet room next to your sims bathroom, that way their baths with soaks shouldn't be interrupted as the toilet will not be in their line of sight.