Hmm... it seems like you somehow triggered an editor bug. The font is rendered correctly in the game itself and it is also rendered correctly if you chose a button instead of a label.
It is just the label, that doesn't like the Xpaider font. Changing other labels to the mentioned font (your debug GUI for example) worked perfectly fine. But creating a new GUI with a new label resulted in a messy font again.
Like you described, I wasn't able to replicate this outside of your game project.
I'd say you also show your sources to Tzachs and Crimson Wizard, maybe they have a clue about what going on.
It is just the label, that doesn't like the Xpaider font. Changing other labels to the mentioned font (your debug GUI for example) worked perfectly fine. But creating a new GUI with a new label resulted in a messy font again.
Like you described, I wasn't able to replicate this outside of your game project.
I'd say you also show your sources to Tzachs and Crimson Wizard, maybe they have a clue about what going on.