Does forwarding a mail learn it as HAM?

Hey all,

I've migrated my aging mail setup to Mailcow and so far it works really well. However, I suspect that forwarding a spam mail to report it learns that mail as HAM, is this possible?

Situation:

I got two spam messages that are largely the same. First one got header-tagged and moved into Junk. I forwarded it (as an attachment in a new mail) to spamcop, left it in my Junk folder, all's well.

A while later the second one arrives and gets put into my inbox. Lo and behold it is marked: BAYES_HAM (-5.5) [99.99%]

From the logs I see that no other user got an identical email, i.e. I can rule out someone else accidentally training it as HAM. (It's a small userbase.)

Working theory: When forwarding messages, the outgoing mail with the attached spam is put into the Sent folder, and that's what triggers the HAM training. However, that seems so dumb I don't believe that can possibly be the actual root cause.

Ideas? Any way to fix this? I'd hate giving up reporting spam; been doing that for, ah, 25 years or so at this point, but I also obviously don't want to sabotage the local spamfilters.