I found the way to reproduce it.
You have to place a bed, do /gamemode survival, then: start holding LMB, before bed breaks stop holding LMB and right away click RMB. You will start sleeping and you'll see your right hand is raised. Position of the hand is just locked when you start sleeping, but it shouldn't.
I actually can't seem to reproduce this anymore.
Let us know if you find a way to reproduce.
I found the way to reproduce it.
You have to place a bed, do /gamemode survival, then: start holding LMB, before bed breaks stop holding LMB and right away click RMB. You will start sleeping and you'll see your right hand is raised. Position of the hand is just locked when you start sleeping, but it shouldn't.
I can confirm, added screenshots.
Can anyone still reproduce this in 1.16.5+?