Following the repro steps posted for UE-12219, I could not reproduce the specific reported issue in that JIRA in UE 4.12.5 with either an Anim Asset or Montage.
If you are reporting a different issue with the AnimMontage not playing, please provide a few more complete/concise repro steps.
I uploaded a project here. Just press P for playing the montage with play rate 1 and L for -1
The result is that P is playing it correctly and with L its directly stopping
inside the project i created a simple montage and added some functionality into the thirdpersoncharacter bp
Thanks for providing the sample project. While this is not the same issue reported in UE-12219, I’ve used this to generate the following new bug report which is publicly visible for tracking purposes: UE-34168.