A single stream is almost the same as a multistream: 1 bit is different in the matype, and the ISI (in this case 171) is supposed to be ignored as there is no need to distinguish between streams.
My theory: Probably the receiver assumes that PLS is only used for multi-streams and does not use it for single stream. If you force it to look for a multistreams it then starts filtering on ISI. There is always an ISI value in the stream even for a single-stream as there is a byte for that in the bbframe and it has to have some value (0 or 171 or whatever). So putting the correct number (171 in this case) tricks the receiver into receiving the stream and into PLS descrambling. There is still an incorrect bit, but the receiver does not notice and the user is happy.
It of course makes no sense to duplicate the muxes, so some will be removed later?