This has been an issue for several versions now, but I did not submit a bug. Every time player 2 joins in split-screen, they will receive damage and be unable to pull items from the full inventory while in creative mode. They also can drown, receive fall damage, etc., but do not have any of the survival HUD. They CAN destroy blocks as in survival mode and can fly; however, they cannot place infinite blocks (only what's in their personal inventory - and they can't drag items from the full recipe book to their personal inventory).  Survival mode and Adventure mode work as intended.

This only affects player two. We have found a work-around for two players by setting up player 2 and 3, then having player 2 quit. Nonetheless, this work-around shouldn't be necessary.

The bug occurred after we had a pressure plate that activated a command block that forced the player into survival mode (for a boss) that inadvertently activated rapidly and repeatedly (the boss was jumping on the button). We deleted the boss, the command block and the pressure plate, as well as resetting, exiting, etc. but the bug persisted. The bug has been in place for several months now, despite our best efforts to reset. I think the character is corrupted in some way, but I can't find a way to delete just one player's data from a single world. That ability would PROBABLY fix our problem.

I suspect it's a rare bug, but it completely ruins 2-player creative mode.