What I expected to happen was...:
After reloading the world, the movement speed attribute was suppossed to keep that value.

What actually happened was...:
After reloading the world, the movement speed attribute value it immediately resets to original value. Definetly not suppossed to happen, since other attributes such as max health are completely fine.

Steps to Reproduce:
1. Use "/attribute" for example like this : "/attribute @s minecraft:generic.movement_speed base set 0.3
2. Reload the world
3. Movement speed immediately goes back to normal