Audiobooks - discrepancy between iOS devices and MacOS device in purchased audiobooks

Devices:

iMac - Late 2012 27", MacOS 10.15.7 [yes I know it's old but read on...]

iPhone - iPhone 15 Pro, iOS 18.4.1

iPad - iPad Pro, 12.9" 5th Gen, iPadOS 18.4.1

Application:

Books

Need:

I want access to all purchased audiobooks on my iPhone which is my primary device for listening to audiobooks.

Problem:

I have audiobooks that are downloaded on my iMac and in my Books library [visible and playable on the desktop] but that are not appearing on my iOS devices. Conversely there are some audiobooks that are on my iOS devices that are not appearing on my iMac.

I thought it might be a synching issue to my iMac since it's old. I confirmed that I'm logged into the same Apple ID on iCloud on all devices. When I compared available audiobooks in my libraries across all devices I find that some older [over 12 months ago] audiobook purchases are available across all devices, some are not. Not all new [within the last 60 days] audiobook purchases are available on my iMac but one is.

So some audiobooks are synching, some are not.

When I look at the Music and TV apps across the devices I can see all purchases, recent purchases, etc. in my Library correctly. Not so with the Books app. Actually there isn't a menu option to show recent purchases in Books.

Things I've tried to resolve the problem:

I tried looking up the media source files on my iMac [Hard Drive > User > Music > iTunes > iTunes Media > Audiobooks > Author] where Author is one of the authors of an audiobook that are visible and playable on the iMac Books app. No media files appear under any of the authors. So where are the media files for the audiobooks?

When I try the same thing with Music or Movies I can see all of the downloaded source files and they correspond to songs or movies that I have downloaded and are visible and playable on the respective Music and TV apps.

When I search the Audiobook Store for an audiobook that appears in my libraries across all devices, it appears in the search results and correctly indicates that I have purchased the device and gives me the option of downloading it. When I search for an audiobook that appears only on my iMac Books library, it appears in the search results but only offers the option of purchasing it. It does not seem to recognize that I have already purchased the audiobook.

I purchased an audiobook on the Audio Bookstore on my iMac. After verifying my account password, it immediately started downloading the book. I checked on my iPhone > Apple Account > Media & Purchases > Purchase History and I could immediately see the audiobook purchase in the list.

Things I haven't tried [for various reasons]

I can't update my iMac beyond MacOS 10.15.7

I haven't tried logging out of and back into my iCloud account yet but the fact that I can see some but not all audiobooks across both platforms but I can see all music and movies makes me think it's a Books app issue or an issue somewhere in iCloud related to my purchases.

There's a very remote possibility that some of the missing audiobooks may have been purchased from another app store [a different country] but there are over 20 missing audiobooks [25%], some of them recent.

Signed,

Stumped.