I don't know if this is an isolated issue. But there's this bug that keeps on happening everytime I restart my S10. The adaptive clock color in the lock screen reverts to a white color regardless of the wallpaper every after restart. Please look into this SAMSUNG. This happened after the latest May update.
Do you use blue light filter? It may happen because any such setting may not be activated soon after the restart. It will be ok after you unlock your phone with password. If the problem arise only once soon after restart, then leave it. May be normal.