Just noticed the "Play" button special character only shows up when the text is selected.
Seems OK here using ▶ as the character:
The text is rendered by Windows, and not really something Opus is involved with rendering at a low level.
It happens that the file display and rename edit field use different APIs, so there can be differences in how they render things. (The APIs used to be consistent but have been diverging in recent versions of Windows, with MS apparently failing to add features to one that they add to the other, for some reason.)
In my case you can see the character is smaller in one place than the other, due to that.
What you're seeing on your machine could be because one API is using font substitution for special characters vs the other one either not using substitution at all or doing it differently. Try using a different font in the file display to see if that fixes things; it's possible the font you are using does not have the ▶ character (or whichever character you are using there) and it either isn't being substituted for another font or that font is saying it has the character even though it doesn't, causing the substitution to be skipped. (This is just a guess. We aren't involved in any of that logic, and just give Windows a list of characters to print on the screen in both cases.)
The file display font I am using is Segoe UI, 9pt.
What happens is also potentially influenced by system locale settings and which languages are installed or enabled.
I've never intentionally changed the Windows 10 default font. The characters displayed correctly on the same Hardware, same OS. Maybe one of the many Win updates changed something.