I've been trying to recreate this for a bit. What you describe makes sense and I thought I'd know what it was. But so far, I'm not able to make it happen. I've tried this:
Post to modern uploading a photo, immediately edit and change the text on modern. That seems to come out right on both sides.
Then I posted to classic (which I think is what you were talking about), uploaded a photo, and completed the post. Then I went over to modern and edited the text. The edit came through but the photos weren't duplicated.
In both cases I used the Advanced posting options "Upload Photo".
What am I doing differently than what you were doing. First step in fixing is always duplicating it.