Is it normal for my boiler transformer to show intermittent drops in voltage?
The voltage drop I saw was due to the triggering of the low water cut off system. When the LWCO kicked on it dropped power to the thermostat. Thank you for the tip @ThreePhaseEel !

Gotta get some of those leaky radiators fixed.

As a side-note, the Ecobee handles the power drop better than the Nest. I tried two different Nest-E units and they both read 5-6 degree lower ambient temps while recovering from the power drop. The Ecobee stayed constant.