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
Related issues
is duplicated by
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I can confirm in 20w46a.
Can confirm in 20w51a.
Can confirm in 21w03a.

Can confirm in 21w11a.
Can confirm in 21w15a.
Can confirm in 1.16.5.
Can confirm in 1.17.

Can confirm in 1.21 and 24w37a.

This is because player motion is entirely handled client-side (The Motion NBT is always the same value for players).
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.