When an image is over an interface like on screenshot 1, they items are not in the foreground, the Count are incorrectly applied, and a white filter is present (black_stained_glass).

and when you target a "Empty" slot this slot does not highlight.

To reproduce :
1- With a ResoucePack create a CustomFont (It's the more simplest).
2- In the Interface Rename The Entity/Block with CustomFont.

You can try quickly with my ResoucePack :
https://www.dropbox.com/s/706bsi7ewnsfthn/Jira%20test.zip?dl=1

team add NameNotVisible
team modify NameNotVisible nametagVisibility never

summon donkey ~ ~ ~ {SaddleItem:{id:saddle,Count:1},Tame:1,ChestedHorse:1,Team:NameNotVisible,CustomName:'{"text":"\\uF808\\uEd99","color":"white"}'}