Stairs, when placed on the corner, have the hit box as if they were a normal stair. Easy thing to miss and from what I can tell, an easy(ish) thing to fix 😛
Included are some images. I noticed it when testing my mod so ignore the stuff in the background of one of the images, but I have been able to recreate the issue in vanilla also.
Related issues
Attachments
Comments

[media][media][media]
Duplicate of - https://mojang.atlassian.net/browse/MC-1307