The first test (replacing Neat Video with Gaussian Blue) demonstrates that the problem is not created by Neat Video itself. It is reproducible with FCPX alone.
The second test indicates that not the clip is bad, but the player software (Quicktime) is bad. The clip looks nicely in FCPX, so the clip itself is fine, but Quicktime shows it badly, which indicates a problem with Quicktime itself.
Taking all these observations regarding FCPX (made by Apple), Gaussian Blur (made by Apple) and Quicktime X (made by Apple) into account, I think it is good idea to ask Apple to clarify the situation.
My guess: the problem is in Quicktime X.
resolve technical issues related to use of Neat Video