if you do shift times of a sub file that has [Fonts] section, that section gets corrupted and becomes a mess. thus the fonts would be ruined so it's kinda of an issue for the files that use them.
Can you upload a sample file? ASS with [Fonts] are quite rare to come by nowadays. It may be related to python-ass, the underlying library used to read and write ASS.
if you do shift times of a sub file that has [Fonts] section, that section gets corrupted and becomes a mess. thus the fonts would be ruined so it's kinda of an issue for the files that use them.