Can't reproduce this here so far. 
Can you please:
a. check whether the output is fine when Internframe isn't used
b. create a small sample video (which allows you to reproduce the issue)
c. create a debug output level 9 of the job processing (using interframe)
d. try if it makes a difference if you use Vapoursynth instead of Avisynth (Vapoursynth also got Interframe under 'Filtering->Vapoursynth->Other->Frame Interpolation')
In case use a graphic card that is overclocked by the manufacturer like the 'Windforce 2X OC', try if it helps to downclock to the default stock values. (VPU chips tend to react unexpected when overclocked and most manufacturers don't test the VPU when overclocking)
Cu Selur
Can you please:
a. check whether the output is fine when Internframe isn't used
b. create a small sample video (which allows you to reproduce the issue)
c. create a debug output level 9 of the job processing (using interframe)
d. try if it makes a difference if you use Vapoursynth instead of Avisynth (Vapoursynth also got Interframe under 'Filtering->Vapoursynth->Other->Frame Interpolation')
In case use a graphic card that is overclocked by the manufacturer like the 'Windforce 2X OC', try if it helps to downclock to the default stock values. (VPU chips tend to react unexpected when overclocked and most manufacturers don't test the VPU when overclocking)
Cu Selur