It works as required in MS 365 on Win 10.
When a particular functions "works" in some places and not in others, trouble-shooting becomes much more complicated. It's not random, but it's not clear what the pattern is. Hence, the method that works best is to delve directly into the object exhibiting the problem. We can't do that for you, unfortunately.
Start by comparing property settings on this, the non-conforming, form and other forms that do work as required. Look at things like Plain Text/Rich Text, for example, and fonts selected for the control. Does a pattern emerge there?
If you add a new control to this form, does the new control accept the character you want?
Can you insert the desired character directly into the field in the table to which this form is bound?
If that trouble-shooting doesn't turn up a solution, try rebuild the form, starting from scratch. Perhaps a new form copying all but the non-complying control and adding a new control to replace it.
It's possible that someone else could spot a problem in your non-compliant form if you share a copy here for review, but you are more likely to uncover the problem with a series of objective trouble-shooting steps designed to eliminate possible factors, one by one.