(it doesn't only happen on realms this is just the example im using) I logged out of my friend's realm with full health, logged in later and i heard the damage noises before the loading screen disappeared. Once i was in i was on half a heart and luckily raised my shield to a baby zombie.

You can reproduce this bug by, for example, placing a block of lava, jumping in and immediately leaving, and you will be dead before you log back in