There is a “known problem” with at least the GTN-650. Transmit on the 650 and something else re-radiates the out of band harmonics and that is what kills the GPS signal (I have first hand knowledge on this). Perhaps other Garmin GPS navigators share this issue - I don’t know.
Here I suggest verifying the GPS antenna feed line meets the minimum required length of the install manual. If so, then you can try not using the notch filter. Watch the GPS signal page when transmitting on your 355 (and any other radio you may have). This will tell the story.
At any rate there is no real down side to keeping this bandaid fix.
Carl