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this is an "aliasing" problem. if you assign like Mat b; Mat a=b; , this is only a "shallow" copy, both Mat's now point to the same pixels.

solution a, clone it:

if ( cnt<= 5) stat_backgnd = small_raw_frame.clone();

(btw, 5 seems to be far too small)

solution b, use local Mat's, not global ones:

UMat stat_backgnd;
capture.read(stat_backgnd);

for(;;){
    UMat small_raw_frame, fgmask;
    capture.read(small_raw_frame);

    pMOG2->apply(small_raw_frame, fgmask); // say what you mean, mean what you say ..

    int cnt =countNonZero(fgmask);
    if ( cnt<= 5000 ) stat_backgnd = small_raw_frame;

    imshow("fg", fgmask);
    imshow("back", stat_backgnd);
    if (waitKey(10) == 27) break;
}