Here is an example of 3 keyframes. They have all transform keys in the same position and nothing has been edited in curve editor.
Now notice how both rotation and quaternion rotation are completely weird and create unintentional rotation. From what I noticed, it seems that you have to rotate an actor on the correct direction to get the intended effect, because yaw/pitch/roll can add up, and then not even quaternion can save it. However, quaternion snaps after a keyframe, or it would face a specific direction in other cases, making it hard to use.
I would love a toggle setting that allows more simple controls and how rotation is determined because I don’t even know what this is.
Also another bug is that once you enable quaternion, you cannot rotate the actor on keys anymore. I know this is a bug because turning the quaternion to off, will allow you to rotate, then it applies once its enabled again. Example: