Prius Prime charge monitor issue

I'm the original owner of a 2017 Prius Prime. I have started noticing that the charge management blue indicator has been getting very low. This is most noticeable when the EV mode is depleted and the gas engine takes over but the blue indicator continues to go lower as seen in the image. I took my Prius into my local Toyota dealership service department and showed them a video I took of the blue indicator dropping lower and they charged me $150 only to let me know they tested the batteries and everything checked out and told me to come back if the problem continues. My comment was "I guarantee this will continue since they did nothing to fix the issue". Can someone remind me why this could be happening? And also, if it does happen what is the best thing to do? TIA