Delay not applied to subtitle comments
Comment lines in ASS files don't get affected by a delay.
I don't know if there's a specific reason for this (I couldn't find an issue for this), but I consider this a bug because comment lines refer to video time just like dialogue lines do, so they should be kept consistent.
For reasons why: Not delaying comment lines breaks their ordering with dialogue lines. Since mkvmerge also moves comment lines to the top, neither line number nor starting time can be used to figure out which dialogue line a comment refers to. Another counter-example are karaoke scripts: The karaoke FX are generated out of the comment time (which is used as the starting time for the FX) and the script in the comment.
Here's an example file. The comment line Comment: 0,0:00:01.00,0:00:02.00,Default,,0,0,0,,not shifted
stays the same after demuxing while the dialogue line Dialogue: 0,0:00:03.00,0:00:04.00,Default,,0,0,0,,shifted
gets delayed/time shifted as expected.