It's not firmware - the issue develops very gradually rather than all at once. This is part of what makes it such an insidious fault, like the boiling frog analogy.
I recently had my pre-Oct 2020 pros replaced with new manufactured stock under this support program. Within less than a month the replacements developed the same sound issues, supported by Apple's diagnosis and another free replacement.
I don't think I use them in an unusual way - I don't even wear them during exercise. There is just something inherently flawed in the design that causes the sound quality to subtly degrade by a significant amount.