I get this annoying problem with iC8. It’s not unique to v8.6, in fact I can’t pinpoint what version it began happening because I originally just thought it was something I was doing.
It’s not a major issue, more annoying. I keyframe animations a lot as I like the precise control. What I’m finding is if I go back on the timeline and click on and highlight a motion key that I have previously created and attempt to edit it through the edit motion layer panel, often it will jump back a frame on the timeline and create another key 1 frame before the key I clicked on, yet the key I originally clicked on is still highlighted.
It’s easily rectified by clicking the key I want a second time and editing the motion layer again and it will work. But it’s an inconvenience when it happens regularly and can be frustrating when you don’t realise it’s created another key. I tend to find out when I play back the timeline and then spot the abrupt change in the motion. I have a look back at the timeline and find 2 keys right next to each other.