If an entity has the glowing effect, part or the whole entity model will render with a solid color based on the glowing effect's color. The whole entity or part of the entity model can also render into the world with the glowing effect (when view at the correct angles).
How to reproduce
Players
Give yourself the glowing effect
/effect give @s minecraft:glowing infinite
Open the survival inventory
Horses
Summon a horse
Tame the horse
Give the horse the glowing effect
/effect give @n[type=minecraft:horse] minecraft:glowing infinite
Put on horse armor
Shift right click on the horse to open it's inventory
Observed behavior
The entity models are fully or partially rendered solid in the glowing effect's color. At certain camera angles, you can also see the entity is being render in the world twice. You can see this better if you change GUI Scale to 1 in Options... > Video Settings....
Expected behavior
The entity models in inventories should have the same rendering behavior as 1.21.8.