If the Display Entity is in the Eye Hitbox in End Portal Frame, then they will be "pushed" out.

Detail Hitbox Range of the Eye part in End Portal Frame:
x,z in range of ~.25 to ~.75 (exclude boundary point)
y in range of ~.8125 to ~2 (exclude boundary point)

What I expected to happen:

Display Entity will stay on the position where they have been summoned to.

What actually happened:

Display Entity has been pushed to an upper location out of the Eye Hitbox.

How to reproduce:

1. place a Command Block with following command and run it:

   execute align xyz run summon block_display ~.749 ~1.813 ~.749 {block_state:{Name:"grass_block"},transformation:{left_rotation:[0.0f,0.0f, 0.0f, 1.0f], translation: [0.0f, 0.0f, 0.0f], right_rotation: [0.0f, 0.0f, 0.0f, 1.0f], scale: [0.5f, 0.5f, 0.5f]},Tags:[test_bd]}

1. place an End Portal Frame on top of the Command Block
2. check where the entity is at with the following command:

   execute as @e[tag=test_bd] run data get entity @s Pos

1. Place the Eye of Ender and check the position again with the same command in step 3.

Screenshots: