It seems that the height of the door frame does not match the size of the characters. When a door is closed and a character goes to open it, their size seems like that of a child in relation to the door.
Viewing post in Zombie Interior - HD Isometric Tileset comments
It does look quite odd in that image. Orthographic viewpoints often cause issues with visual scale, since objects farther away don’t appear smaller as they would in a perspective view. Have you tried adjusting the character’s scale slightly? in the video you can pause when a character moves thorugh a door to see the difference.
I tried changing the character's scale, but for other objects like cars, boats, beds, and tables, the character's current scale is correct.
The problem really only concerns the door and window frames. It's as if I had to reduce the scale of the walls instead of increasing the character's scale.
In your demo video, looks like you has the same problem.