See attached .afdesign file. Each icon on the canvas is 38 x 28 px.
Selecting pano1 and exporting it via "File > Export..." , either as a PNG or a JPG, generates an image 59 x 43 px.
Now do the same with pano2, which is a copy of pano1 but without arrowhead. Designer in this case generates an image 39 x 29 px. Still 1px too large, but pretty close.
The only way to get the correct size after exporting is to make the box size much larger but to leave the arrows at their original size - which is precisely what I don't need.
It always happens so it's reproducible. I am using the latest MacOS version and the latest Designer version.
nav_4.afdesign