The code for the number of NAL units in the first DU for picture timing SEI message has a "-1" missing.

The line of code (in TEncGop.cpp, HM-9.1-dev)

pictureTimingSEI.m_numNalusInDuMinus1[ i ] = ( i == 0 ) ? ( accumNalsDU[ i ] ) : ( accumNalsDU[ i ] - accumNalsDU[ i - 1] - 1 );

should be

pictureTimingSEI.m_numNalusInDuMinus1[ i ] = ( i == 0 ) ? ( accumNalsDU[ i ] - 1 ) : ( accumNalsDU[ i ] - accumNalsDU[ i - 1] - 1 );