I was just searching for a similar issue. I'm assuming this is the same as I'm experiencing?
Basically, I have just created a note with a particular tag, then if I filter by tag, the note never shows up along with the others I have created previously (older notes were with the older Evernote 6.x). I also have a saved search set up with the same tag, and it doesn't appear there either.
Windows 10
v 10.3.7 build 0 public
Editor: v111.0.14414
Service: v1.22.6
Just tried the same on my Mac, with the same issues.
MacOS 11.0.1 (Big Sur)
v 10.0.10 build 458340 public
Editor: v107.3.13847
Service: v1.20.21
Hope this info helps.