There is a UI bug in Media Composer 8.5 and 8.6. It was reported to me by my users and I've done extensive testing to confirm the details with a clean build and new user and project settings.
If you apply an image effect to a sequence (e.g. Mosaic or Paint effect) and have the shape highlighted, clicking the 1 frame step button forward or backward (or using the keyboard shortcut) actually causes the shape itself to move around the screen instead of stepping the video forward or backward. The odd thing is it does not happen with the 10 frame skip button, which behaves properly. If you deselect the shape, the 1 frame step buttons work as expected.
This bug did not happen in MC 8.4.