You've massively detrained. That's all it is. Most likely. Do you have other symptoms, like muscle or joint pain? Overcooking it always results in a lower HR during hard efforts. Continued attempting to force a high HR with hard efforts while HR remains unusually low results in overtraining if continued long enough. Resting HR OTOH will increase by as much as 10 beats during the process of becoming overtrained. The difference between resting and resting standing HRs will also increase during this process, becoming as high as 20 beats or so. It's really easy to tell what's going on from these markers. They are very reliable. Upon quitting training for a long period to recover, HRs will very slowly come back to sedentary levels. Resting and working hard HRs both will come back to higher than trained levels, which is what you're seeing. Normal.

Afib is very noticeable with an ordinary strap-and-watch or Garmin type HRM.

I think when you were doing recovery rides at under 100 HR, you were already overcooked. I never do them at less than 105. When I'm totally overcooked, I can hammer the pedals without HR going over 105. One week completely off, then resuming at a reduced level fixes that. It's good to find where your limits are and learn to recognize them.

As I've mentioned many times, it's good to take both one's resting and resting-standing HRs and keep track of them and the difference between them, one's orthostatic HR.

I think it's possible that your MHR now is higher than it was at previous sedentary levels because you still have some aerobic ability in there.