Just noticed this morning that some of my submissions in my submissions list had "comments" next to them without the number of comments. After clicking on comments and refreshing, or clicking on comments and going back to the submissions list and refreshing, it showed the number of comments next to the "comments" link.
I doubt it's a bug. PG is probably just minimizing what's cached outside of the most current and/or most viewed submissions. If a user views it, then it gets cached (for awhile, at least). Just guessing though.