Hi Tim_Wulteputte welcome to the forum
AFAIK, “locked” means and does just that. A member locked at TL2 will not be promoted to TL3 even if they meet the requirements. Conversely, a member locked at TL3 will not lose TL3 even if they do not meet the requirements. Similar for TL0 → TL1 → TL2
If this is not the case for you, it could be a Bug.