I think it might be a BUG in Unreal 4.20, i replicated the problem on a default flying Project from Unreal, now you can replicate and see what the problem is.
To replicate: create a default flying Project, open flyingpawn BP, connect Event Begin Play to “show mouse cursor”, now to enable mouse, go to “Project settings” create 2 axis mappings, name them “turn” and “lookup”, set turn to “mouse X”, and set look up to “mouse Y”, now return to Flyingpawn BP and make this configuration for “apply forward speed and steering rotation”, it has to be like this:
Now, as you can see, the problem even occours at a default flyingproject from Unreal, i belive this is a bug at Unreal 4.20, since all stepts you mention before are OK at this default Project, even mouse for touch its disable.
While you flight, move cursor, space ship Will follow, make a click and ship Will stop following the mouse movement, i haven´t found a solution jet…