Some challenges might have a small lock icon (similar to the Challenge by Status) with a Key:

The time in which the lock is activated changes between types of challenges: it can appear once the challenge opens or even the last few hours.

Currently, all challenges lock in the last 24 hours apart from "Mega Prizes"/24H challenges which are locked from the start (please note this may be changed).

If a challenge will lock in a time that is different than the last 24 hours, it will be stated in the challenge details.