emptyDisplayValue with HTML not correct in combo box

I've been searching the code for hours now and since I haven't found the solution yet, I was hoping somebody on the forum could help me out.

When I create a SearchForm with a SelectItem in it with a emptyDisplayValue containing HTML tags ('<em>Product</em>'), it shows the title italic (which is what I want). When I do the same with a ComboBoxItem, it doesn't.

I've seen a similar question about this, but it's not completely the same: I only want to have the title italic (to keep it consistent with other form elements, including SelectItems that do work).

Is it a bug, or can I set/override a property/method?