There is now a quality stat for fish. The better the quality, the more the quantity when converted by the cauldron.

whats the number on the fish the fish now have ratings like deer, boars and necks? a 1 star troll fish wont stack with a 2,3,4,5 star troll fish.
the more stars the more fish you get when converting them in the cauldron.
if you mean that it won't stack with different types of fish(I.e perch, Tuna) then that's just common sense they are separate items.
if its anything other than that likely a glitch/bug which suggests a validity check. and a report to the devs to test and potentially repair.