Hi,
Even before the recent updates others have reported kerning problems with some iMovie titles, particularly the Line and Lower Third titles. I'm not aware that Apple has fixed that issue yet. I have not seen any previous reports that the text automatically changes. The reported changes have been kerning and line jumping issues upon adding text and/or changing font sizes.
While it has not worked for others with this problem, you can try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see of the kerning issue is resolved.
-- Rich