The bug
When the "Text Background" setting is changed to "Everywhere", a gray rectangle is displayed behind items of text such as those above the hotbar. Unlike most UI elements, this cannot be retextured.
How to fix
Introduce an image file to handle rendering this.
Linked issues
relates to 26
MC-278414
Subtitle background cannot be retextured
Open
MC-278413
Chat background cannot be retextured
Open
MC-268805
panorama_overlay.png no longer works
Resolved
MC-268927
The dark background in inventory screens cannot be changed via resource packs
Resolved
MC-269079
Death screen background cannot be retextured
Resolved
21 more links
Can confirm.