When an attribute modifier is applied in a piece of armour and the command is put on a clock (/replaceitem) the attribute is effective only after re-joining the world
MC-99569
Attribute Modifiers on armor is buggy
Resolved
Invalid
0
Could not reproduce. If you have multiple commands applying different sets of armor, you must have the UUID pair be unique across all items and modifiers. They must be universally unique, as the name suggests.