Right now, a parameter inlay hint is automatically omitted using a series of heuristics defined here. This isn't referenced in the documentation at all, and while useful, also cannot be tweaked or disabled.
As a result it can appear that inlay hints for parameters are broken, since quite frequently these heuristics will match and they will be omitted.
I think an appropriate resolution would be to both document the heuristics being used (something I'm planning to PR in a moment), and to allow disabling these heuristics. Ideally each heuristic could be individually disabled and/or configured but a global disable for the heuristic omission would also be ideal.