In Text bit_depth_chroma_minus8 is supposed to be alwas present when in HM there is a condition for monochrome and it seems to be proper:
encoder:

if (chromaEnabled)
{
    WRITE_UVLC( chromaEnabled ? (pcSPS->getBitDepth(CHANNEL_TYPE_CHROMA) - 8):0,  "bit_depth_chroma_minus8" ); // NOTE: RExt - This SE is not in the SPS header for 4:0:0.
}

decoder:

const Bool bChroma=(pcSPS->getChromaFormatIdc() != CHROMA_400);
if (bChroma)
{
    READ_UVLC( uiCode,    "bit_depth_chroma_minus8" ); // NOTE: RExt - This SE is not in the SPS header for 4:0:0
}