I found similar issues, but they didn't seem quite the same. I have contained dolphins in minecarts, and put them on top of a 3 x 3 grid of magma blocks, on the center block. I also make sure they are not adjacent to any chunk borders.  Sometimes they live for a while, but unknown duration. When I check the holding area I find a raw cod, so I know they died instead of despawning. The minecart has not moved. All the water blocks above the magma blocks are water source blocks for 2 levels. In version 1.14 they never died.  They take no damage that I've ever seen while watching them in containment.