Why does iTunes keep deleting my home movie artwork?

Why does iTunes keep deleting my home movie? I'll upload a home movie into iTunes, enter all the info and add artwork (often the movie poster) then click okay. All the other info remains but the artwork never takes. However I do have older home movies (added before iTunes 12.2.2.25) where the artwork is still there. But anything added since won't take. My home movie library is on a external drive connected to my network.