The Bug:
The "minecraft:movement_speed" attribute resets upon reloading the world.
All attributes should be saved upon the world being reloaded. A workaround for this bug would be to use attribute modifiers instead of altering the base attribute.
Steps to Reproduce:
Alter your "minecraft:movement_speed" attribute by using the command provided below.
/attribute @s minecraft:movement_speed base set 0.3
Run around and observe your movement speed.
Exit the world, reload into it, run around, and observe your movement speed.
Observed Behavior:
The "minecraft:movement_speed" attribute is reset.
Expected Behavior:
The "minecraft:movement_speed" attribute would not be reset.
The bug should be assigned to the Commands category.