In TDecSbac::parseQtCbf(), there is this piece of code:

  if ((width != height) && (lowestLevel || !canQuadSplit)) //if sub-TUs are present
  {
    const UInt subTUDepth        = lowestTUDepth + 1;
    const UInt partIdxesPerSubTU = rTu.GetAbsPartIdxNumParts(compID) >> 1;

When the total number of partitions in a CU are calculated, the sub-TU partitions for 4:2:2 chroma are not taken into account. This means that when decoding, for example, an 8x8 TU when log2_min_transform_block_size_minus2==1, rTu.GetAbsPartIdxNumParts(compID) will come out as 1, which will give partIdxesPerSubTU=0, causing the decoded CBF information for the sub-TUs to be discarded (stored across a block of length zero).

This doesn't happen when log2_min_transform_block_size_minus2==0 as the 4x4 chroma sub-TUs are then at least one layer above the lowest luma layer, so sufficient partition indices are available to store the data.