Read only field unlocked

Hi,

Always in the same spirit that fields appearing or not depending on custom plugin setup, when i switch from a status to another one, knowing that for those 2 status the field "Charge" is supposed to be in read only mode, how can you explain that this field become updatable when i select the new status in DDL, but after saving become read only again ? It is risky as the user is able to update the field between the 2 status change...

I hope to be clear enough.