Blizzard has acknowledged that since the 1.15.4 patch for Season of Discovery Phase 5, there has been a bug in both Season of Discovery and Classic Era where any enemy that is 3 levels higher than the character will show as a Skull, rather than their actual level. The team is working on a fix for this issue, but it could take several days to resolve.
Season of Discovery Phase 5 Patch Notes
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.