It maybe related to MC-168139 or even a clone of it. If being far away from an entity with an enchanted armor piece, the glint disappears. It might also happen in certain angles or probably related to what things render in the background.
To reproduce
Run the following command:
/summon minecraft:piglin_brute ~ ~ ~ {NoAI:1,PersistenceRequired:1b,ArmorItems:[{},{},{id:iron_chestplate,Count:1b,tag:{Enchantments:[{id:mending,lvl:1}]}}]}
Move away until it disappears
→ ❌ The enchantment glint is gone.
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Can confirm in 21w03a.

As of 1.19.1-pre1 this seems to have been fixed.
Can confirm.