It’s just a math problem. 
As per the C++ you deleted: You can’t simply turn a FRotator to FQuat and expect it to show different results. Quaternions are complicated to say the least, probably the reason why it’s so hard to find info about them.
Think this is what it would look like if the axis don’t flip:
If you could be more specific about the range of motion that you need, I’m sure it can be done without complicating things further. You only want the actor to pitch; no roll nor yaw?