I was delighted to read Matthew's (Member mbooher) report that "as of right now (17:15pm Pacific Time on Saturday, August 3, 2019), EVERYTHING SEEMS TO BE WORKING AND NORMAL AGAIN".
With hope in my heart, beginning about 18:05:00 PDT Saturday, August 03, 2019, I ran my send/receive/sync tests again.
As I too am in the Pacific Time Zone, I thought I'd be served by the same or similarly updated Verizon "integrated messaging service" servers as Matthew's devices.
I am disappointed to report that my results have NOT changed.
The 8x8 matrix of results of my Send/Receive/Sync Verizon Messages conversations (SMS text) remains unchanged.
Exactly the same send/receive/sync results as in my original spreadsheet matrix (appended below).
I then spent (WASTED, as Verizon is not paying for my time) additional hours on my Windows 10 Home laptop: Disconnecting/Rebooting/Connecting, reviewing Conversation threads in, Uninstalling/Rebooting/Installing, and again reviewing Conversation threads in Verizon Messages (v126.96.36.199).
During these cycles, the Conversations were "deleted" and "reloaded" from the servers and the same "last successfully synced message was still 7/19/2019 - 2:11 PM PDT".
When I sign in to my Account on Verizon's website and go to Account : Text Online, ALL messages subsequent to 7/19/2019 - 2:11 PM PDT are there (see appended screen capture), including the tests I just ran, inspired by Matthew's post.
Matthew (Member mbooher), might you be willing to try sending a message from your Samsung Galaxy Note 9 or iPhone SE (whichever is your Primary Verizon cell number) to a cell number different from yours, receive a reply back from that other cell number, and then check if your Lenovo ThinkPad Yoga 260 correctly synchronizes both sides of that Conversation thread?
May we assume that your ThinkPad is updated to a recent/current version of Windows 10 Pro?
If that does work, perhaps we can discover how your environment differs from mine.
If your situation has actually changed from "unable to keep in sync" to "working now", I can hold on to some hope that my situation may still get resolved.