This might be intentional. Catching fishes in a bucket will release the fish but will empty the bucket instead of cloning the captured fish and leaving the bucket filled with a fish. Probably because there was an issue where the tropical fish mob changes to a different type, which is covered in MCPE-37329.

How to reproduce

1. Grab a fish bucket and a water bucket from the creative inventory
2. Use the fish bucket on the ground ( You keep the fish bucket)
3. Use the water bucket on the fish
4. Use the newly captured fish bucket on the ground ( It reverts to a normal bucket, unlike the first time)