Do magnets interfere with my DC circuit?
You have a typical string EMI problem that can be shunted with a 10nF to 100nF.
Anecdotal
I just two reels of 12V RGB LEDS and Blue was dim whether it was On or Off using the IR remote control. fortunately I bought two and it was a floating gate triggered by my hand and body picking up E fields from line.
Recommendation
The most reliable will be an Optical to transistor switch either reflective short range or interrupt type. With black daylight blocking filter included.