That change isn't significant. Vaccines in the past have not had 100% effectiveness, and we all know that. There've been mumps outbreaks that were primarily among the vaccinated, despite that vaccine having a very high effectiveness. The reason they were primarily among the vaccinated? Vaccination rates were well over 90%, so even with a 90% rate of success the odds favored some getting the illness.

Cases were mild as the vaccine provided the expected protection. This is not new.