Exported images become smaller.

[ChildDream]

ver 1.8.5.703
Win 10 ver 1909
Exported images become smaller.
Affinity Designer, Photo, and Publisher have the same problem.
01. Create an artboard by using Affiniy Designer.
02. Import a photograph to the artboard.
03. The layer structure is like this.
04. Switch to Affinity Photo to use the live filter. 
05. Apply the noise reduction filter to the artbord.
06. Export the artboard.
07. This is the exported image. The photograph becomes smaller with a strange margin around it.
If I don't use the artboard, I won't get this problem.
(The description number is the same as the attached images.)

[MEB]

Duplicate of this thread.