Antenna interference

Help me to understand. I was just assigned a 2025 mack anthem, identical to the 2019 I got out of, got tired of running my own coax and antenna, figured I’d use the stock stuff which I know isn’t impressive. Either way, went to the truck this morning and routed 18 feet of coax out to my browning antenna like I always have, but I left the stock mack antenna in the mirror, I’ve heard it’s theorized that one of them is AM/FM receiver, while the other is the CB antenna, I kept the drivers side stock antenna removed on my old truck, I didn’t on this one. Keying up in the lower channels, up to 17, was throwing an antenna light, I removed my stuff, going back to stock, I left the browning on the mirror and keyed up with the stock setup, same antenna light, removed the browning, no light. So long winded question is, why would the stock antenna being in place and running my own coax and antenna cause an interference in performance. I’ll attach a pic for reference of my 2019, my own stuff with the stock antenna removed on the drivers side, I’m guessing I need to do the same thing now because that set up never gave me an issue, help me understand why though.

 

Sorry for the dated pic, thanks for the answer though.