This happened to me, OP. My stutter was pretty much gone for 20 something years and then came back during Covid (I’m in my late 40s). I still had a stutter as an adult but it was mild and didn’t affect anything (for instance, I worked at a call center for a few years with no issues). But then during Covid, I started having issues again. Especially on the phone at work. I went to speech therapy again and the therapist said he’d seen other cases like mine where childhood stutterers were stuttering again. It’s better now but it still gives me issues daily. I don’t usually suffer from feelings of anxiety and tend to have low distress but I think the collective anxiety from Covid and masking all the time might have contributed to stuttering again (for me that is). The therapist thought so. It’s seems to be wired in our brain like you said. I’m glad you are making drastic progress.