Description of the bug:
When a player moves the mouse over an item, whose count is shown, if the tooltips are activated, the count itself is shown on the same graphic level as the tooltip itself

How to recreate the bug:
Enable tooltips using the f3 + h key combination
Give yourself a number of items (can be any item) whose Count is greater than 1. Use for example the following command:
/give @s glass_pane 2
Hover the mouse over the item, going to the leftmost part of the item

Demonstration of the bug in game:

 I attach a picture of what happened in the game.