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Most likely it's because we fixed the upvote value calculations and those accounts don't have enough pending vote value to power comment upvotes anymore.

Comments are supposed to be upvoted at only 4% of member pending vote balance, but it was originally released at the same 20% as posts receive. Once we fixed that, any accounts that were in between the two thresholds would stop getting upvotes while their balance continues to grow. They will start getting upvotes again once the balance is high enough to support them (or if they post instead of waiting for comment upvotes).