I was flying when my elytra ran out of durability, at which point I fell out of sky like a classic cartoon character.  After a long walk home and evaluating my life decisions, I mended my elytra.   The method I used was a dispenser full of Bottles o’ Enchanting, powered by an observer clock, while holding the tattered elytra in my hand.

As soon as the XP hit, the elytra graphic in the hot bar updated, but not the one in my hand. Fair enough.   However, i changed items in my hand, and the elytra’s appearance remained tattered.   I equipped / unequipped the elytra and it remained tattered.  I tried “manually” equipping it in my inventory screen, and it remained tattered (in hand, mind you, not the inventory icon).  I dropped the elytra, picked it back up, it still appeared tattered in hand.  

i even picked up a completely DIFFERENT set of Elytra, and it too appeared tattered.

the only workaround was to relog into the realm, and that fixed it