Hello. Since this is an old topic, it may already be solved for everyone involved, but since I ran into the problem today and found this thread, I'll post what I've discovered. It seems, at least in some cases, that if you have the "auto-filing" feature turned on in your Evernote preferences, it could override the "+" command. I learned this a few minutes ago when I tried tried testing the feature by creating a Test Note and forwarding a random work email with the same subject and "+."
Except the random work email never appended, and never even appeared in my default notebook even though I never used the "@" comment to specify another notebook. Turns out, the random work email was re-routed to my work notebook (and was assigned a couple tags)... When I turned off the "auto-filing" feature in my preferences and tried again, it worked!