While working in UI Toolkit in Unity 6 imported fonts eventually have issues with spacing.
Upon first importing the font it will be fine and spacing will be fine. Eventually, however, something goes awry for the font itself, even if you have not edited anything related to that font or text. All text using that font will experience spacing issues for anything more than 1 word.
I believe this happens to ALL imported fonts (Have had 3 do it so far). Possible causes are changes to any element of UI, repeated saves of UI Document or time - as my project is heavily UI based and UI Toolkit is opened for long periods and both saves and alterations are frequently done by me and text unrelated to these changes eventually gets ruined.
This renders that font unusable without error in any label field afterwards for anything more than 1 word, and spacing issues happen to what appears to be random characters for each font.
Example: This font spaced correctly yesterday. Today while working I noticed that an area in the background that used this font had incorrect spacing. I made another element visible that used that font and spacing for that was also wrong. I previously had this happen to another font, which required me to change the font used elsewhere.
*Reimporting does not fix font. Unable to fix font once this has happened.
** Creating a Font Asset for these fonts and using that seems to be the only fix.