Since the release of 1.15.4, there is a known bug in WoW Classic Era and Season of Discovery where, if an enemy is more than 3 levels above your level, it appears to be ‘skull icon’ level. In original WoW, the skull icon only appears for raid bosses or enemies that are 10+ levels above you, so this is incorrect behavior. We’re working on a fix, and intend to correct this as soon as possible.
Enemies are still the same level they were prior to the patch. Enemy levels have not changed.
We do not yet have an ETA for the fix, and we expect it to take a couple of days, or more if we need to apply it during maintenance.