Armor and some items not appearing on some mobs when given to them via commands

When putting armor on wither skeletons, the armor is invisible but it is affecting the mob (takes longer to kill, etc). Same issue when wither skeleton was given a bow. Endermen holding any items that aren't blocks also have the items invisible but they are still there (gave enderman a diamond sword to test and it gave the sword but it was invisible. I know this because when provoked, the enderman one-hit killed me when they normally take half your health.) I would assume this issue applies to other mobs as well besides wither skeletons and endermen (had a vindicator hold a totem in the same hand he was holding an axe in)