When logging out, all momentum the player has is deleted, and the player will spawn completely still when relogging. This can especially be seen with Elytra & Creative flying as well as Ice sliding
Linked issues
is duplicated by 4
MC-201039
Elytra loses speed after relog
Resolved
MC-271798
Player's velocity isn't saved after relog
Resolved
MC-278869
Player Velocity Always Reset When Reloading World But Fall Damage is Not
Resolved
MC-280536
Speed of elytra flying set to zero wrongly after quiting and reentering the world
Resolved
I expanded the report to include the general issue of players losing all momentum when relogging. Note that I have only tested this in Singleplayer at this moment.