Alright, ticketing this because I externally checked with a friend who confirmed it's not just a visibility issue on my end:
If someone else makes a post with 1+ images (via the Attach An Image option) and some text, and I reply to this post with 1+ images (via the Attach An Image option) and some text, when the share goes live only OP's part will be visible. The entire body of my reply will be gone as if I'd never made additions at all when viewed: in the Latest Posts feed, on my own page, or via direct link. The tags I put on the reblog will be visible, the text I put in my reblog will be reflected in the reblog's URL, and if I Edit the reblog all of the images/text/etc will be there.
I tried asking a friend if they could see the reblog additions and they said it was not visible in their feed, on my page, or when reblogging my reblog.
To make this somehow even more puzzling, I have a confirmed case where someone reblogged my reblog anyways and then my post in the chain appeared in the feed, on their blog, etc:
This has now happened on a second post. I confirmed with a friend that my reply on the reblog chain was completely not present at the Edit view, but when their reblog went live it appeared in feed view, on their blog, and the direct post link. They have further stated that if they go back into Edit This Post view my reblog isn't there.
All of my text-only replies (that skip the Attach An Image button) seem fine, no problems with them showing up. I don't know the OP in either of the above cases so any mute/ban/etc behavior is presumably not in play.
I do have Cohost Plus, although the image files in question are much smaller than the default free image size limit.
All of this is happening on Firefox 107.0.1 64-bit (desktop Win10 PC).
Addendum that for the second example (rainbow gifs) someone reblogged the FROR (incrementing the chain to four) and my reblog additions vanished again.
I am the soul of confusion.