Most creatures in D & D 5e have details about what attacks or types of weapon damage are resistant. For example
Resistance to damage: Aggress, pierce and graze from non-magical attacks
However, the description of Flameskull carries resistance to drilling damange without the phrase "From Nonmagical Weapons / Attacks":
Resistance to damage: lightning, necrotic, piercing
–Flameskull, Manual of Monsters, p. 134
Does not that mean that the Flameskull is resistant to magic piercing damage as well as non-magic piercing damage?