Summery:
If using a command with [distance=..1.5] for example it will not detect some the entity even if you standing in its hitbox. (see images and/or video)

It's detecting only if the player/command is <1.5blocks from the bottom of the entity you try to detect.

Expected:

I expect it to be even detecting the entity across its whole hitbot.

Steps/commands to recreate:

1. spawn a Endermann wich no AI: /summon enderman ~ ~ ~ {NoAI:1b}
2. stand next to it at the bottom and use: /execute if entity @e[type=minecraft:enderman,distance=..1.5,sort=nearest,limit=1] run say test
3. Fly up to its head and run the command again.

Note:

Having it across the whole hitbox commands like with [distance=]in it, would be more accurate.

I personal use a raycast system which should detect a mob if it's get hit by the raycast, but facing the raycast at the eyes of the mob it will not detect some entity if they're to tall. I could fix this by increasing the distance inside the command but then it will hit something else in some situations.

Having the detection-zone across the whole hitbox, I could lower the the distance and it will be more accurate using such commands.

I couldn't find an existing bug report for that, if there is already one I missed, I'm sorry.