In about two of three cases the realm crashes when I use a nether portal in any direction while followed by a cat. When reconnecting, the world is reset to a state about 30mins ago (probably the last savegame). All progress after this point is gone. After experiencing this bug a few times, I tried to force a savegame by disconnecting and reconnecting to the realm before passing a portal with a cat, but this doesn't work at all.

To reproduce, just tame a cat, let it follow you and go through a nether portal, most times this will crash the realm and corrupt the world.