Is it possible that a secondary antibody, such as anti-IgG1, binds to an antibody on the MACS MicroBeads?

The antibodies on the MACS MicroBeads will also bind to secondary antibodies. The corresponding data sheets always state the antibody isotype, so you can avoid this. For control staining, we do not recommend that you stain the antibody on the MicroBeads, because the staining intensity will be too low. In most cases, only the smaller part of the epitope is covered by the MicroBeads, so even a staining with the same clone is possible. In cases where this will not work, the protocol indicates that you should use an antibody that recognizes a different epitope.