I am posting this mainly to document the problem. I am not currently using the Evernote v10 Windows client. Problems in various Evernote clients and versions led me to go back to Evernote Windows Legacy version for some uses and to switch to OneNote for most active new work. FWIW I've been an Evernote premium user since approximately 2008.
My account is still "premium." I have not upgraded to one of the new account levels.
After not having used the v10 Evernote Windows app for several weeks I cranked it up yesterday to see whether it is working. It showed me notes only up through August 8 (I'm posting this on August 27). The Web Evernote, the Evernote legacy app, and the Evernote Android client had notes up through the current time and showed about 400 more notes than the v10 app showed. (The new articles are web clippings or email newsletter message copies. I'm not actively manually creating and editing new text notes in Evernote.)
I uninstalled and reinstalled v10 Evernote. (Yes, at some point I rebooted my PC but I don't remember exactly when in the sequence.) The version that I installed and am currently running is v10.20.4. I left things alone overnight and this morning. Then I manually entered a new note via the v10 Evernote client/app on my PC.
The new note sync'd to Evernote Web, Evernote Legacy, and Evernote Android. Evernote v10 continues not to have sync'd any other notes at all from after August 8.