When an item_display, block_display, or text_display entity mounts another entity, it does not change its position. Instead, it visually behaves as if it's not mounted.

How to reproduce:

Run the following commands in order.

summon minecraft:item_display ~ ~ ~ {item:{id:"minecraft:stick",Count:1b},billboard:"center",Tags:["test_passenger"]}
summon minecraft:pig ~1 ~ ~ {Tags:["test_vehicle"]}
ride @e[type=minecraft:item_display,tag=test_passenger,limit=1] mount @e[type=minecraft:pig,tag=test_vehicle,limit=1]

Observe how the item_display entity, which looks like a stick that is always facing the screen, does not follow the summoned pig's position.

According to the game, the item_display entity is riding the pig. This can be shown with the command

execute as @e[type=minecraft:pig,tag=test_vehicle,limit=1] on passengers run say hi

which outputs the message "[Item Display] hi" in chat.

The expected behavior would be for the display entity to either mount the other entity as normal and follow its position, or for an error message to appear when attempting to make the display entity ride another entity.