I don’t keep the DAC on but you will have to admit this is a head scratcher
Um, no it isn't!!
Anything with a computer or DAC generates some RF. Your particular situation is just more noisy than others. I'm not sure why this is a puzzle, really. What's probably happening is that the RF from the DAC is broadly across the channels you are trying to tune, and it's lowering the quality of the signal lock.