First thing I’d do here is to import the FBX back into Max to see if the problem lies with the exported animation. If so, then at least you know if it’s an exporting problem or actually just in UE4.
So fixing it, one thing you can try, unless you already did, when you export try and check Resample All. Though I found it’s generally better to keep it unchecked, but it’s something you can try. I have had one or two cases where checking it produced better results, compared to unchecking it. If you currently have it checked, try unchecking it then in stead.
The other thing you might try, which is basically what I do with my faster animations, is to give it more frames, so it’s kinda slower than it ultimately would be. So lets say your move is 3 frames total where you create the arc of the sword slash and that is the speed you want. That is quite extreme for a full arc, in stead make it 5 frames… or whatever. So you cover more points on the arc with actual key frames. Basically slow everything down by a little bit. Then in UE4 you can just speed it back up, but you have more frames to cover the poses and less chance of interpolation error.
I have to admit that I have never seen this happen with my own animations, but I have created them with this (slow down) method from the start, because I know game engines do their interpolation and I wanna be sure to get something accurate, so it’s possible I just dodged the problem like that. Or who knows, it might be something totally different creating that problem that I have not come across before. So I just wanted to mention my thoughts, maybe it helps you out.