Tiles that are destroyed and fill with water will capture a pacu and not let them move. The pacu become stuck.
I can Alt+Q to move the pacu to a safe location and they continue swimming; until they path to the same block and become stuck again.
It looks like it may depend on how the tile is filled as I destroyed 8 blocks, but they are only getting stuck on 3. I'll update this post if I do some additional debugging. See update below.
The pacu's pathing will update when the game is reloaded (ie, reloading the save is the current workaround). This also means the save file is a bit useless, but I am attaching it anyways. Thanks Klei!
There was one reference to this behavior here in this bug report:
== UPDATE ==
Turns out this happens when you destroy tiles in the downward direction, not the upward direction. Two screenshots below show the layout before, and then after I destroyed the tiles in sequence.
Build a pacu tank with some solid tiles in it. Destroy the tiles so that they fill with water. Wait until the pacu swim into the now filled tiles. Watch as they cannot escape.