I have the same experience with P. Palustris. After they transformed emerged growth to feathery submerged growth, they stopped growing and dropped lower leaves. I failed multiple times with this plant which is difficult and needs strong light.
My first lemon Bacopa melted away which I blamed on bad stock. I’m trying it now the second time and hopefully it will work this time. From what I read, lemon Bacopa is an easy beginner plant, and can be used to make lemon tea.
The B. caroliniana smells more like menthol to me. I'm sure it has some medicinal value. They do great with CO2. I have to trim them every week or 2.
It's interesting to see emersed growth where they grow to a certain height while they grow infinitely in water.