The bulbs work fine at 5.5 vdc. They blow at around 8 volt.
But from earlier discussions they look like they will get 3V per bulb in normal use, that is 3V RMS which is a peak voltage of 4.2V. You would be better off just running them at 3V.
The black ring in the glass makes me think they are blowing from a voltage spike and not over voltage.
There is no correlation and you can not draw that conclusion. In fact an incandescent bulb is much more resilient to voltage spikes than just about any other component.
I would also think they would all blow at about the same time if it were from over voltage.
That would be wrong. Any component driven over the design rating will fail at random. The more over stressed they are the more often is a failure, but they do not fail at the same time unless they are well over the rating. You are in the grey zone which is not enough to trigger instant death but too high for reliable working.