Though it has been a very long time, and worse there was practically no communication, it appears that this problem is now fixed.
First clue was that all the messages (well for most of my contacts, for possibly that 90 day maximum backup window) were duplicated on my phone. What a huge pain, I had to manually delete the duplicates. [The same stupidity happened when I tried to do a restore on my phone, because of the other problem I reported, where I had messages deleted.. Verizon told me to try and Restore from the Cloud, that it would really restore more than the 90 days worth... Instead it only restored 90 days worth, and I now had duplicates of everything. Arghh... but I digress.]
Anyway it seems that messages are now correctly syncing between my Samsung S7 phone and the Windows 10 PC app Verizon Messages app. I did install the most current/recently released update to the Verizon Messages (Message+) on my phone (Dec 27, 2019, Version 6.9.11), maybe the fix was there.
In any event, Happy New Year to me! Thanks to whomever was involved in fixing it, and fingers crossed it stays that way forever more!!!