ICE continues to run when stopped with traction battery fully charged

I couldn't seem to find a post that aligned with my experience, so I'm sharing here with hope someone else may find it useful. Please let me know if I've missed a thousand threads that already cover this.

Yesterday evening, I noticed that the engine continued to run at red lights despite the battery icon showing it fully charged. There also seemed to be some hesitation when accelerating from a stop.

During some recent maintenance, I noted that the my 12v battery displayed 11.9 in the Function/Check Setting>Vehicle Signal, despite being just over two years old (DieHard Hybrid Battery). When I Googled my symptoms, I read that it may be related to a failing 12v, so that seemed a reasonable starting point …

This morning, after the car sat overnight, I checked the voltage on the 12v with a multimeter and got 12.3. Interestingly, I checked the Vehicle Signal menu immediately following and it still said 11.9.

This afternoon, I had the battery tested at Advance Auto. They said it passed the cranking amps test but failed the "reserve capacity" test.

I installed a new battery this afternoon. I will report back if the original symptoms return or if the 12v battery seems to have been the culprit.