The bug
When you hover over an entity name in chat, it should show the entity UUID and the entity type.
How to reproduce
Say something in chat and hover over your username. No hover text appears. (the click event still works)
execute the following commands:
/summon minecraft:armor_stand ~ ~ ~ /say @e[type=!player,limit=1]/summon minecraft:armor_stand ~ ~ ~ /say @e[type=!player,limit=1]hover over the entity name; no hover text appears
This can be closed again, apperently it is influenced by the Advanced Tooltips setting