Frames marked as valid returens non valid data

[neomimiz]

Hi, 

I working with eye tracking data that I collected with Sr anipal and Unity 2019.4 a few months ago. I have noticed that frames which are marked as valid sometime  have non-valid data values (a series of zeros or -1 values). 

I attach here a figure exemplifying this issue. In this figure there are histograms of data values of  gaze origin and pupil dilation. All data values included in these distributions have validity of four or greater than four.  (which according to -https://www.frontiersin.org/articles/10.3389/fpsyt.2020.572938/full , should provide valid data).

What is the reason for this issue?

How can I solve it for next time using Sr anipal?

Thanks a lot in advance!

Neomi