Your message got me to thinking and I suspect that Outlook's built-in spam
detecting rules are superceding my rule.

Since a spammer starting spoofing my return address, I'm getting buried by
"bounced" messages: about 50/hour.

Outlook's internal spam detector is moving these messages to a "Junk"
folder.

However there are so many of them, that when reviewing the "Junk" folder, I
can't spot the handful of other email that sometimes is flagged as spam but
isn't.

I'm trying to create a custom rule that would automatically move these
messages to a "Bounced" folder rather than the automatic "Junk" folder.

So, I think your analysis is correct: another rule (Outlooks internal spam
detector) is getting to these emails first. I'm going to try turning off
Outlook's "Automatic" filtering (as much as I hate to) to see if my rule
then is applied. At least then I'll understand what's going on.