The bug:

When summoning an interaction entity with a height other than the default value (1), the blue hitbox line gets placed up to about 5/6ths the interaction's height.

You can flip between versions 1.21.1 and 1.21.3 while trying the same tests. With this bug, even typing a height with as little difference as 1 and 1.0001 makes the blue line completely shift upwards.

How to reproduce:

1. Enter any world with commands enabled.
2. Execute the following command:

   /summon minecraft:interaction ~ ~ ~ {width:1,height:1}

3. Execute the command from step 2 again but change the value of height to anything other than 1 like 1.0001, or 2.
4. Notice the inconsistent placement of the blue line when using F3 + B.

Expected result:

No matter what height you set the interaction to be, the blue hitbox line would stick to the bottom of the interaction, or the top of the interaction if the value of height is negative, just like in previous versions.

Observed result:

Unless both width and height are set to 1, the blue hitbox line appears around 5/6ths the interaction's height above the bottom of the hitbox (or top if the value of height is negative).