Herzlos

I'm using a Garmin Forerunner 35 (pretty basic running watch but it does what I need), and a Garmin Speed Sensor 2 on my gravel bike.
When I run it on the turbo trainer with the GPS off, I get pretty reasonable figures for speed/distance. All good so far.

I left the GPS on for a trainer session (you configure it independently of the rides) and found that the speed sensor was showing 0 or near 0 for large chunks of the ride and assumed it was broken until it uploaded a scatter map to Strava.
It seemed that it was sometimes using the GPS speed (0) and sometimes the sensor speed (correct) but I can't see any particular pattern to it beyond it maybe relating to the points where the GPS thought I was moving.
Turning the GPS back off and the speed sensor is fine.

Does anyone know how the Garmin decides which one to use?

I want to start taking the bike back out on the roads, so should I be OK to go on the road with the GPS and speed sensor or should I take the speed sensor off? I'm aiming to use it outside at weekends and during the week on the trainer so I'm expecting it to change from indoor to outdoor and back most weeks which will be a bit of a pain with the sensor, and though I'm tempted to get another bike just for trainer use I can't really justify the space or money.