Strange mirroring behavior

I've only been using Cascadeur for a day or so, so this issue is probably me just not understanding some basic concept. But the mirroring function does not work as expected. First of all I imported an FBX of DAZ Genesis 8, if that may have anything to do with it. I used the quick rigging tool to map the basic joints to the Genesis bone system. So far so good. The figure is absolutely usable in Cascadeur. Not with all the original bones since the quick rigging only maps some basic bones. But, it's good enough for my purpose. I just want to make a simple walk animation with four keyframes. Which is typically two keyframes followed by the same two keyframes mirrored. So the initial setup before any mirroring is keyframe 3 being an identical duplicate of keyframe 1, and keyframe 4 being a duplicate of keyframe 2. I then go to keyframe 3, select all the joints (double clicking on the hip/pelvis), and select Mirror Tool > Mirror on current frame. And it looks fantastic. And then repeat the process on keyframe 4. Which also looks fantastic. Except now keyframe 3 is mysteriously un-mirrored. It's back to being a duplicate of keyframe 1 again. Attempting to mirror keyframe 3 again however un-mirrors keyframe 4. There is seemingly no way to mirror both keyframes. It's as if every time you use mirror tools on a frame, it's somehow being cached or flagged as mirrored. So the next time you use mirror tool, any previously mirrored frames will be mirrored again (back to original pose). I've been struggling for an hour to simply mirror two frames but no success. I even tried to make temporary keyframes in between but no difference. Lastly, I tried Mirror Tools > Mirror on Interval. Which only mirrors the last keyframe. I feel like I am doing something very wrong here. Is there anything I need to click or apply to make the mirroring stick to a keyframe?