Reducing the player's movement speed with the the Slowness effect and negative minecraft:generic.movement_speed attribute modifier will result in extremely high movement speed.

Steps to Reproduce:

  [Step 1]Run the following commands on the player:

/effect give @s minecraft:slowness infinite 9 true

/attribute @s minecraft:generic.movement_speed modifier add test_modifier -100 add_value

  [Step 2]Move around. Run the next command to check the movement_speed attribute value:

/attribute @s minecraft:generic.movement_speed get

Observed Results:

The player will move with extremely high movement speed. The output from the command in [Step 2] will show a high value of around 49.95.
For reference, the unmodified movement_speed value of the player is approximately 0.10.

Expected Results:

The player SHOULD move not at all / move an unobservable amount. The output from the command in [Step 2] SHOULD show a value of 0.0.