Gordon I am not sure if my situation is similar but hoping this may help.
I was able to import Evernote (EN) ENEX files of EN Notebooks fine into Apple Notes (AN) on the Mac BUT noticed the sync up to iCloud.com stopped working. Plus edits or new notes on the Mac were no longer going to iCloud. New notes on the iPhone were being sync'd back to the Mac just fine though.
I contacted Apple support and we established at least one of my EN notebook imports would stop iCloud upload to work. No errors of any kind show - it just stopped working.
To get AN back to syncing ... in the Mac settings we unchecked the iCloud setting for Notes (careful as this will lose and with no warning anything that hasn't sync'd to iCloud.com). And then rechecked the setting. Takes a while to download from iCloud but once done we checked that new notes were making it to iCloud.com.
We repeated the problematic EN Notebook ENEX export imported and again verified the upload to iCloud was broken.
Did the uncheck/check process to get AN working again.
For the same problem notebook. We tried using the EN multiple web pages (.html) export and that worked into AN and the sync to iCloud.com was working!
So the trick seems to be NOT to to import using ENEX format and rather multiple web pages (.html) export.
Unfortunately in AN the notes now the EN notes have today's date rather than the original EN date. And there are some formatting differences. And pictures seem to import correctly but it does NOT seem like PDF attachments do - on a test they were missing.
Weird because some EN ENEX notebooks worked AOK with AN and iCloud sync. But at least one broke it and with no import errors or warnings from AN!
