While this is true, this does not appear to be the issue with the OP. It is with one airline, and not one that does a lot well.
Also, since I enrolled in Pre-Check, I have never had a flight where it did NOT show up. I have had SSSS once (CDG, where Pre isn't applicable, but was still listed) and occasional random secondary screenings, but 100% of my flights (I fly often and have been enrolled since 2018), every boarding pass has had it, including the one last month on a carrier I have not flown in 20 years and my main carrier interlined me on to get me where I needed to be. I had my KTN in their system too, of course, but it just showed right up like it does on all the OW carriers. So no, not guaranteed, but it's unusual not to--and ATL. UGH! Sorry, OP! Fly AA or even DL. (Better upgrades with AA since DL basically owns that airport).