I have noticed a number of errors with the alternate hero skins when in Classic graphics (i.e. neither Reforged nor Classic HD). The ones I have observed thus far are:
*Cairne Bloodhoof (Tauren Cheiftain): Portrait camera shows a random camera angle.
*Varimathras (Dreadlord): In-game model displays as the standard Dreadlord rather than the alternate model used for Varimathras, Balnazaar, and Tichondirus in the classic campaign. Portrait displays the correct model, however.
*Kel’Thuzad (Lich): Uses the standard Lich soundset rather than Kel’Thuzad’s unique soundset, and displays standard Lich names instead of being named Kel’Thuzad. Portrait is broken as with Cairne Bloodhoof.
EDIT: Found another one: Kael (Blood Mage) uses the the correct sound set, but displays the generic Blood Mage hero names.