I have the same issue. Message+ v6.9.0 on ZForce and the Verizon Message App v18.104.22.168 on 2 Windows 10 desktops. Messages that are sent and received always show up on the phone, but many of them never show up in the Win 10 apps, leaving odd gaps in conversations. I even uninstalled the old Message+ app from the Win 10 desktops and installed the "Verizon Message" app from the Win store and I'm having exactly the same results.
I see that this has been an issue for some time now. I've had this issue for several years now and decided it was time to speak up. I'm very disappointed to see that this has been reported to Verizon for years now with no apparent action taken on their part.
Please advise as to when your development team will get around to investigating and fixing this problem. It makes the messaging functionality far less valuable when it misses messages all the time, making the Verizon service far less valuable by extension.
Thank you for your attention to this matter, if in fact it receives any attention at all.