For any significant mutations to persist, they would normally have to give the life-form at least some small improvement in ability to survive and reproduce in its current typical environment. So an observed change may be ultimately due to some consistent change in the typical conditions it finds itself in, which in this case could be something to do with modern treatment regimes.

Day to day evolution has little or nothing to do with advantages or survival or improvement. It can result in these, but it won't necessarily. Viri often become less capable of infection due to mutation.

It is only over the long term that evolutionary changes tend to the advantageous, as the advantaged continue on pace or become accelerated, while the disadvantaged become stressed or even extinct.

In short, there will inevitably be new strains of any virus that are less dangerous; and some that are more so. The weaker strains may persist long enough to be noticed, but they are unlikely to persist long enough to weaken the virus overall (unless it also strengthens the virus' survivability).

I do not believe that this virus is evolving to become less effective. It is far more likely that researchers simply stumbled upon a strain before it faded away.

Every time we see life reproduce we see evolution.