I am in the same boat and need an assist. Here is my situation:
2011 imac (Lightroom Classic) and 2019 MacBook Pro (Lightroom CC). A series of external hard drives and backup drives. Additionally, I run LR CC on my Google Pixel and on an Android Tablet all synced thanks to the Adobe Cloud.
On my iMac, I import all my images and they are saved on an an external hard drive while sending backups to a second external hard drive.
I create collections and sync to the cloud.
Once in the cloud, I manage and edit my images on any of the following devices: Macbook Pro, Android Phone, Android Tablet or sometimes in LR Classic on my desktop.
When I complete my edits, I go to my iMac (desktop) and access the synced collection. From there, I export and am able to get the original file resolution since LR has access to the hard drives.
My current situation
I've been working on a few projects recently and was using mainly LR CC on my Macbook Pro. I rated and had completed edits. I was ready to export and went to my iMac to start the process to get the full resolution images. When I got to the computer I kept running into a low disk space warning for the partition I was using. But no problem, the new update was out and High Sierra wouldn't support it anyway so I decided to install Classic on my Macbook Pro and transition my classic workflow to my laptop. I was due.
I did that with no issues and dropped my catalog on an external drive. I then opened the catalog on the LR Classic Installation on my Macbook Pro. It made me update the catalog which wasn't an issue. It then started syncing my pics down from the cloud. All seemed fine until I realized that my recent edits on the cloud did not sync to LR Classic. They show on all my other devices but no matter what I do, I can't push the edits to LR Classic.
I am trying to avoid having to redo all of the work. Additionally, I created a test collection in LR Classic and place a single photo. That album nor photo appeared in LR CC.
So to summarize: LR Classic sees the albums in LR CC, but edits/ratings, etc are not traveling in either direction.
Are you seeing any metadata changed icon (in Grid View) on images in any of your synced Collections?
In Loupe view, many commands only operate on the most selected photo, not all the photos you selected in the filmstrip. Do the menu command View > Grid to go to grid view and then do Metadata > Save Metadata To File -- the command will apply to all the selected photos.