I used to get that with Polar near tram tracks in Melbourne or sometimes near HV power lines. Perhaps your rides are taking you near something with large electromagical power whereas your runs are not.
I was riding on a trail in the middle of a golf course, no HV lines in sight. Could be some Illuminati / Bilderberg shit going down in their countryhouse?
My Garmin occasionally reports heartrates in excess of 240bpm, only when cycling not when running, both of them alone. What could be wrong?