If I pass a note to the external editor (I use Typora) and I actually edit the text, the image is broken upon return to Joplin. It does not matter where I insert it, in Joplin or Typora, Joplin does not see it.
I don't know about pre-existing images, here I saw this on the images I paste from the clipboard. Also, I did not check other external editors, so it may be specific to Typora, but since it is one of the best MD editors, it deserves further study.
version?
This was 1.2.8 I think. Then I saw your post about 1.4 and installed it. There, external editing is just broken - no outside changes register in Joplin.
same problem in version 2.4.12