i have had instances where copying/pasting/mirroring an ik control have caused weirdness in other frames of animation, but usually, just moving the scrubber one frame then back, or by switching selected actions, the anomaly disappears.
however, in this particularly stubborn instance, mirroring the center pose of this animation (specifically, the leg ik controls) causes the leg positions to be messed up in the start and end frames. not only that, but this error propagates to all the actions in the file (which use the same base starting pose). and, yes, it exports like that.
this video shows what is going on: https://youtu.be/GFtKuuj6jdA
blend file is here: https://sta.sh/025kacu1mhbg
it has empties in it to show where the start and endpoints of the toes should be.
your mission, should you choose to accept it, is to make this 'turn left' animation into a 'turn right' animation.
i have a workaround for it, which is to delete the start and end frames, mirror the central frame, then MIRROR paste the start pose from a different file. but it is driving me nuts because i don't know WHY it is doing this, HOW to make it stop, and HOW to avoid having this problem in the future.
thanks for looking! yes, it's 2.79. if it works right in 2.8, let me know!