Health App Cardio Readings After Getting a Pacemaker

Hi everyone. I have been using the Health app on my iPhone and Apple watch for several years to track my cardio fitness (VO2max, Recovery rate, etc). I'm a 70-year-old non-athlete who walks/runs every day.

Last year I developed arrythmia and had a two lead pacemaker implanted in January. I had a month recovery period when I didn't exercise much, but once I started exercising again, the Health app says my cardio fitness is trending steadily downward.

My running times and endurance are back to where they were before I got the pacemaker, but the watch says my cardio fitness is 25% worse and getting lower each day. I've been researching this for a few weeks, and I've found papers that describes Apple's general method, but not in enough detail to understand why I'm getting these estimates. The pacemaker has "decoupled" the lower heart rate from any sort of exercise or recovery behavior, so that's probably part of the reason.

Has anyone else had this problem? I'm really missing the "Cardiac Fitness" accuracy, because it was motivating to see it going up as I continued to exercise. Now, it's trending down no matter how much I exercise.

Any suggestions appreciated.

I'm using Apple watch model A2774 with OS version 10.5 (21T576).

Steve