Hope everything is well! That is definitely some interesting behavior you are encountering. To my knowledge, I do not believe there there is a tooltip limitation within parameter-based calculated fields. My guess for why Total rows is working is because it is technically counting that as a separate aggregation, and maybe it is having trouble interpreting with the parameter context.
If possible, could you please send me an Arena View with no sensitive data, as I would like to see more in-depth why this could be occurring and potentially finding a workaround from there.
Thank you!