In my game -- no. 113363821 -- I played a Fregate on a couple of turns, after which, on their turn, my opponent played action cards. Specifically, they mostly played the Overlord as a Coven or as a Mystic. Additionally, they were often starting their turn with extra cards drawn from a Stowaway played on the previous turn. And yet, at no point did I see them discarding cards. The only benefit I was getting from playing the Fregate was the +3 coins on playing the card.
The Frigate worked, but unfortunately the log is not explicitely calling out Frigate as the source of the discard :(
The first time (their turn 11) they played an Overlord (reducing their hand size to 4), playing a Mystic and guessing wrong, so their hand size was 4 at all times when Frigate checked, so they didn't have to discard.
The next time (their turn 16), they indeed started with +2 Cards from Stowaway, then again played Overlord --> Mystic, but they did have to discard (because they started with 7 cards after the Stowaway):
(https://i.imgur.com/qlU9093.png)
There is nothing on Overlord or Mystic that would explain the discard, it is the (unmarked) effect of Fregate.
The last time (their turn 22) basically the same happened, they got +2 Cards from Stowaway and had 6 cards in hand when they were done with playing Ratcatcher, they had to again discard 2 cards. They played Mystic and Stoaway afterwards, but since that did not increase their hand size, Frigate did not apply again.
I am really ashamed now to have given you all that trouble. I can't believe I have not noticed those discards in the log: I was actually looking for them. But, indeed, it was late and I had to be running. I thought I'd better report this before I lose the game number. I'll make sure to be more careful in the future. Apologies again.
No need to apologize, that happens, to me as well sometimes, when I apparently don't see what's before my eyes. I do poke the coders about the attribution, because many of the new effects come at "surprising" times.