This is not really a bug but it is rather a possible shortcoming of lossless coding at CU level.

If a CU needs to be encoded losslessly, but the residue is zero, cu_qp_delta cannot be transmitted. One may force the encoder to use another prediction mode in order to get a non-zero residue (just for the sake of transmitting cu_qp_delta) but there may be no prediction mode with any residue. In that case the block cannot be treated as lossless (for instance because DBF will be active).

Conversely, if a block needs to be encoded in a lossy way, but the predicted QP is zero and there is no residue, it is not possible to indicate that lossy coding is active and that we want the block to be filtered.

One solution can be to enforce lossless coding at slice level only, or, if one wants CU lossless granularity, to have a slice level flag that allows lossless coding in the current slice, and then, if this flag is active, an extra CU level flag for lossless coding.