Did some investigating, seems like the uibutton uses the "Segoe UI Emoji" font when the text is a UTF-8 symbol.
I also see the 'all-black' version of the fast-forward button when I enter char(0x23E9) in the command window. If you copy and pase this into a Word document the symbol changes to the blue version you are seeing and the font is listed as "Segoe UI Emoji". Interestingly though, if you enter char(0x23F5) in the command window and copy that into a Word document you get a 'tofu' symbol.
Based on some google searching it appears that the Segoe UI Emoji font doesn't have a supported icon for the 0x23F5 symbol but it does have one for the 0x23E9 symbol. I imagine when MATLAB attempts to 'render' the 0x23F5 it is defaulting to another font that does have a supported symbol. Having said that, it's unclear why MATLAB is forcing a font other than the 'Monospaced' that you have declared.
I think the easiest solution is to use the 'Icon' property of uibutton to instead display images of these symbols.
