Adding animations with a negative play rate (playing backwards) or modifying existing animations in a montage to have a negative play rate causes the timeline in the montage to "squish" and is no longer functional.
Another separate, but related AnswerHub report: https://answers.unrealengine.com/questions/153035/461-anim-montage-cant-play-animation-backwards.html#comment-158989
Open a project that contains a montage and supporting anim sequences.
In the anim sequence change the play rate to -1.0
Open a montage that contains the anim sequence
Observe the anim sequence in the montage timeline.
The sequence will appear "squished"
Head over to the existingAnswerHub thread and let us know what's up.