Mvc range validation - working only upon clicking submit

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MVC5

I have two MVC edit forms where range validation works perfectly, highlighting as soon as you click away from a text box containing an out-of-range value.

But I have a third edit form where, for some reason, range validation is only working when I click the submit button, but not when I just click elsewhere away from the out-of-range text box. Any idea what might cause that?

I have the Ranges, like [Range(0, 4)] specified in a supplemental partial class to avoid the auto-generation. I have the range text included in the edit view form, like:

@Html.ValidationMessageFor(model => model.G, "0 (Lowest) to 4 (Highest)", new { @class = "text-danger" })

What I have tried:

Not sure at this point what to try. The faulty edit view appears to be set up the same as the good ones.