Since the route and distance are still being recorded, I’d check whether this is a speed-metric issue rather than GPS. The SE 3 does support average speed for Outdoor Cycle workouts. → Apple Watch SE 3 - Tech Specs - Apple Support
On the iPhone, check Settings > Privacy & Security > Location Services > System Services > Motion Calibration & Distance and make sure it’s on. → Calibrate your Apple Watch for improved Workout and Activity accuracy – Apple Support
I’d also start a short Outdoor Cycle workout and watch the current speed while riding. If current speed works during the workout but the finished workout still shows 0.0 average speed, that would point more toward a Fitness/watchOS 27 software issue than a GPS problem.
Since this started immediately after the update, I wouldn’t reset or unpair the Watch yet.