Steps to reproduce:
obtain 2+ buckets of tropical fish or pufferfish (unknown if other fish buckets would be affected)

Use one of the buckets on a block to place the water and fish

Observed results:

The bucket used on the block does spawn a fish, and turns into an empty bucket. However, any other buckets containing fish turn into plain water buckets, which do not spawn fish when used. 

Notes:
Was playing on a server, but was only one on was not experiencing lag. Has reoccurred once, this time turning a bucket of cod into a water bucket. No error messages or anything shown. Seems to just delete the fish from existence.