1. Open Asset Forge 1.3
2. Choose 'cube' from the 'primitives' group
3. Place a 3x3 grid of them
4. Colour the cubes (this shows the issue clearer than plain white)
5. Click '2D Sprite' in the top right
6. Choose 'Isometric' from the camera preset
7. Choose "Large' as the sprite size
8. Export

I have literally just done this now and got the exact same results as I posted in my OP.

This is the result in the preview window:

This is the result zoomed in a little to show the lines:

So it is a reproducable bug, but only on PC?

Thanks for testing/confirming the problem NottheDan, appreciate it,

Has nobody else come across this issue before and is anyone reading this willing to do a quick test?

Is this software still being updated / bug fixed?

Seems Kenney doesn't post here much anymore.

Is there an ETA on that, and is it an upgrade or a new paid version?

It's kinda unusable for me at the moment when anything I export has seams on the exported images.

Has this been announced?  I can't find anything about it.
I'm surprised there are no other threads on this issue as it makes 2D sprite exports basically unusable.

The issue with the lines on 2D exports or that there is a v2 in the works?