iOS 18.4 - manual audiobook transfers losing cover art in Books app

It seems that iOS 18.4 may have broken something that - after taking quite some time to be fixed - was properly working under iOS 18.3.2.

Namely, since iTunes' retirement, the only method I'd found to manually transfer audiobooks to an iOS device (without the use of any form of syncing) and to be recognized within the Books app was to manually drag the .m4b audiobook file into the iPhone's open Finder window. The .m4b file would be correctly recognized in Books, with its respective cover art intact (as well as any embedded chapter markers).

Just attempted this same similar method of transferring a manually-created (i.e. not purchased through Apple Books storefront) audiobook under iOS 18.4 - as much as the file itself transfers and is recognized in Books (with its chapter markers set), it will not transfer with any cover art metadata tagged within the file.

Even if the original audiobook file is imported into Books for Mac (where artwork was retained) and re-exported (assuming Books resizes/rescales some oversized artwork to spec), this new file still suffers the same fate in Books for iOS where its cover art is lacking.

Going to log this as a bug feedback - if anyone else is now noticing that manual (i.e. non-purchased) audiobooks are losing their cover art in either the main library or the widget, I would do the same.

(there was a longstanding bug with the widget not displaying artwork from imported audiobooks that was finally addressed back in some iOS 18.3.x build - wondering if this bug fix didn't move forward during 18.4 testing)