Last Friday, Evernote support reached out to me, and they reindexed my account on the backend. They also provided a link (https://help.evernote.com/hc/en-us/articles/33813205622547-Clear-local-data-for-effective-troubleshooting) for instructions on reinstalling Evernote.
After clearing cookies on the web version, the sync function returned to normal. I used Revo to completely uninstall the desktop version and then reinstalled it, which also resolved the sync issue. However, I noticed a bug: when I remove thumbnails, the sync issue reoccurs, and I need to repeat the above steps to restore functionality. As a result, I am currently avoiding the "Remove Thumbnail" feature.