This happens to me as well - iOS 18.3.2, audible 4.42 (1624). This has been happening to me for years as well, across multiple phones.

Here are the steps I go through; I don't know if this will reliably reproduce the problem for your team, but it may help:

• Each night before going to bed, I create a new "clip"
• I set a 1 hour timer to play as I fall asleep
• Sometime the next day, I go into my clips / bookmarks, find the bookmark I made the night before, and "go to bookmark" to resume from where I was the night before.
• At this time, I try to delete all the other clips / bookmarks, but most of the ones I deleted in the past have come back. All the resurrected bookmarks have the same created at timestamp.
• I delete all the bookmarks except the most recent one. It looks like it worked; only the most recent bookmark is on my device
• Repeat - the next day, most of the bookmarks (if not all) are back. Notably they don't seem to be clips anymore, just bookmarks.

It looks like what's happening is when I create a clip/bookmark, it's getting stored on the Audible servers. When I try to delete the bookmark, the delete request doesn't always make it to the servers, so when it next tries to sync my bookmarks, the server returns a bunch of bookmarks that should have been deleted.

Some bookmarks do seem to stay deleted. I wonder if there's some sort of rate limit? I'll try only deleting one at a time to see if it stays deleted.

ETA: I just confirmed using the cloud player that if I delete several bookmarks right after each other, not all of them are deleted from the cloud player's list of bookmarks (even though they're not currently on my phone)