I have several COSMO-4W (4 wire) combination smoke & CO detectors connected to my M1 via a COSMOD4W interface module. When I issue a smoke reset, I don’t get a CO alarm. However, I do get warnings for both the fire and CO zones as these zones are defined as supervised with 2200 Ohm EOL resistors and interrupting the power to the COSMOD4W interface module causes the smoke and CO zones to show as open (i.e., 13.8V) until power is restored. Once power is restored, those warnings clear automatically after a few seconds.
If you are getting a true CO alarm on a smoke reset, it sounds like you’ve got your CO zone type defined as “Normally Closed”. When you do a smoke reset, the M1 interprets the open CO zone as violated. Assuming you’ve got 2200 Ohm EOL resistors on the CO zone, try defining the CO zone as Type 0 (i.e., EOL Hardwire).