I noticed this when using a teammate's high-level Savage One, while my own Savage One was just level 1.
After completing a Guild Raid, I received a notification (red dot) that my Savage One could equip new items. The hero was displayed as being level 7, and had several green "plus" icons notifying me that I could equip those items. However, when hitting the equip button, nothing occurred. I suspected that it may be due to a display bug, so I applied a single XP potion. This resynced my client's displayed XP with the server's, and all was well.
Fairly low priority bug, but should be a simple fix

Account ID: 22523814