I bet it is because of an issue of upload, not emailing, and those images aren’t uploaded to forum at all. It happends to me all the time, if that batch is someway too big. I don’t know if there is lack of some unicorns, cores, RAM, cache, swap… but it happends and everytime when images are too big, like directly from a phone. That has been an issue all the time.
And has happened here too when I tried, something like a year ago — but I had something more relevant to do and I didn’t bother start a bug topic, If I’m remembering right 
Because nowadays I’m uploading smaller images or smaller batches (max 2 images) I can live with it.
And yes — that can be a new issue too, of course.