How to reproduce

1. Use raw JSON text to name a mob. For Example:

   /summon minecraft:creeper ~ ~ ~ {CustomName:'{"text":"E.G.","hoverEvent":{"action":"show_text","contents":{"text":"TEXT FOR E.G."}}}'}
   

   → You can see nothing happens when the mouse hovers over the summoned message and killed message. Only named text and mob info.

2. Do the same thing in naming item. For Example:

   /give @a minecraft:diamond_axe{display:{Name:'{"text":"E.G.","hoverEvent":{"action":"show_text","contents":{"text":"TEXT FOR E.G."}}}'}}
   

   → You can see the edited info when the mouse hovers over the message. Not the item info.