I've noticed this as well. For me, once I exceed 16383 vehicles, Watch It! simply continues displaying as a percentage out of 16383. It doesn't update to display the capacity to 65535.

So, for example, if I have 21576 vehicles, Watch It! will display it as 131% or 132%.

So for vehicles the max will be about 400%, and for parked vehicles the max will be 200%. It will still show the actual number of vehicles on the road or parked, even if it exceeds the vanilla capacity. So it doesn't affect the vehicle counter, just the calculations for expressing the number of vehicles as a percent of the new capacity.

It doesn't even change the limit number :D:D:D
But at least we know that More Vehicles work in this case.

It is on my list to investigate this.