I also experienced a deterioration in the devices for a few months this year, although this could not (only) have been due to the firmware, as the phenomenon occurred independently of this. In my case, the ANC was much worse, so that activating it only led to a change in the perceived ambient noise, not to the previously very good reduction. The buds were also slightly quieter.
This is no longer as pronounced, but still not as good as it was at the beginning.
If you were not satisfied with the sound from the start and compare them with other buds (i.e. not with over-ear headphones), your buds may also have a different problem.