Summary:
If the generic movement speed is modified with the attribute command, it will reset to the default value again after rejoining the world.

Steps to recreate:

1. /data get entity @s Attributes[1]     ( to see the current value )
2. /attribute @s minecraft:generic.movement_speed base set 2     ( Set it to 2 )
3. leave and rejoin the world 

Now the Player has the default value again. (can be checkt with the first command again)

Only happens to the movement speed attribute.

Expected result:

The player should keep the set movement speed after rejoining.