In building an underwater stronghold, I captured 4 squid and contained them in a 5 x 6 x 3 "aquarium". The floor and one wall were stone with two other sides glass block. There were 3 levels of water.
When caving 30 block below the stronghold, I "found" 3 squid. When I returned to the stronghold, the aquarium was empty.
Really sucks as I thought I had something unique and was ready to share with the Minecraft world.
The "bug" is allowing the squids to "phase" through solid blocks.
Happy to provide further info if you need it.
Larry