[G0HZU_JMR]

It's possible the PA stage in the CB is going unstable when you present a certain antenna load to it and it may be producing a large out of band spurious signal. This signal would have to be able to get through the output LPF so it might be going unstable at a nearby frequency where your antenna has a very poor VSWR. As you change the coax length you change the impedance presented to the PA so this might affect how strongly it goes unstable.

CB amplifier stages are generally well behaved but I vaguely recall that the PA device is an MRF479 in those old radios and these have been obsolete for a long time. You may find that someone has fitted an alternative type of transistor (or a fake one) and it may be more prone to instability. I also vaguely remember some grounding wire link issues with these old President Lincoln radios.