Strange MMT real time preview

[avan1]

 I am trying to remove the star with MMT/MLT  on a linear image with  a mask  covering the background,  without any adjustment the real time preview  just  show a few  black  patch.
   I am running PI 1.8.8-2 on Kubuntu 19.04 and win7  , both platform show the same black patch.
 The size of my image is 9663x4130 ( default 32b xisf,152.3M ).If I crop the image and use the central part 3409x2401 and process with MMT , the preview will back to normal .
   any clue

[avan1]

 same dimension (9663x4130) RGB 32b xisf has no problem with MMT preview...

[avan1]

problem solved 
just add o.1 to the image , no more black patch