The bug
The fox hitbox is incorrect.
The hitbox does not correspond to the size of the fox (it is too small in depth and a bit too large in width).
The red line that normally shows where the fox's eyes are, is not in the right place when the fox is sitting (too low).
The hitbox does not adapt when the fox sits (it is too small in depth, a bit too large in width and too small in height).
Examples
[media][media][media]
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Related issues
relates to
Attachments
Comments

Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w07a. Feel free to use the following commands to make reproducing this much easier:
/summon minecraft:fox ~ ~ ~ {NoAI:1b}
/summon minecraft:fox ~ ~ ~ {NoAI:1b,Sitting:1b}
/summon minecraft:fox ~ ~ ~ {NoAI:1b,Sleeping:1b}
Can confirm in 1.17.
Can confirm in 1.18.2 and 22w11a.
Can confirm in 1.19.
Can confirm in 1.19.2.

Can confirm in 24w45a. Also looking to request ownership to maintain the report 🙂

Just a note: depth and width cannot be changed separately on hitboxes, as hitboxes don't rotate along with entities. For that reason a size in between the desired depth and width is selected so that the hitbox is neither overly small, nor overly big; the better result is in between.