SPST RF switching application with PIN diode
Sounds like impedance mismatch and standing waves between your SPTS circuit and the antenna. So, technically, not mainly your circuit's fault: You didn't match your monopole antenna.

A monopole by itself does not have a 50Ω feedpoint impedance (50Ω is a very common wave impedance); your patch antenna is most probably already matched to your system; you can adjust a patch antenna itself to do that.

Just because your patch antenna works without external matching doesn't say anything about any other antenna, and especially not for the monopole, which is known to be mismatched to 50Ω.

So, add a matching network to your monopole antenna.