Have had this problem basically since Warbands came out, but it does come and go and change up now and then.
Basically my characters, displayed on the Warband screen, can become “corrupted”, glitching out in a number of ways:
-A character’s texture can become pure black, or a mangled mess of colors, textures… I have seen it where multiple characters have a Haranir’s skin/fur texture wrapped around them weirdly.
-A character’s gear can display as a different version. For example, for PVP gear it might show as Red on an Alliance character. Raid gear, sometimes just one shoulder, can display as a different raid tier, like LFR instead of Mythic.
-Sometimes I think I’m seeing dozens if not hundreds of spell or ability icons wrapped up in a character’s skin texture.
-The load times in general for Warbands can be drastically higher than they should be or have been in the past. It is worth noting that I have almost a full account, around 65 characters, and I don’t use addons.