I found some additional info I wanted to share with everyone on this. After upgrading to 4.7.4, I tried removing my fix from above and played a montage with a anim instance with a -1 play rate and a start time of 0.6667 and end time of 0 and played that montage forward. This still crashes on line 152 of Animation\AnimCompositeBase.cpp. So I’m currently not seeing the Epic changes to animations helping this issue.
After putting my fix back in, I actually found some similar issues in other parts of the same file causing crashes. I wasn’t handling cases of root motion among other things. I applied more fixes and I now appear to have fixed the issues I was having. The little preview window in the montage isn’t updated correctly, but it appears to work in game ok.
If anyone else is having similar issues, you can try my changes in here. They are all marked with // CombatJack Begin…// CombatJack End comments. This is based on version in the 4.7.4 branch.