Answer
The client actually does perform this validation visually (an invalid value gets a red underline), but if the user proceeds anyway and clicks Next/Finish, the invalid input is simply dropped/ignored rather than blocking the report or surfacing a clear validation error — the report then runs without that parameter value rather than throwing an error the way the older product did. This inconsistency between the visual validation cue and the actual submit behavior looks like a genuine bug rather than an intentional behavior change, so raising a support case referencing this exact symptom is reasonable if it matters for your reports.