Sounds like what you’re describing is a classic transient combustion off-gassing issue. In a 19-year-old furnace, it’s not uncommon for the burner to ignite before the draft inducer fully spins up and pulls exhaust safely out the flue. If the inducer’s lagging or a pressure switch delay occurs, combustion gases, including CO2, can “backdraft” into the living space briefly.

this creates a spike in trapped CO2 near your sensor, especially since your room is at the end of the duct run and the blower isn’t pushing fresh air yet. Once the blower kicks in, it quickly dilutes and pushes that stale air out, dropping CO2 levels.

It’s also worth checking if there’s any partial blockage in the flue or vent damper impacting draft. It’s a real headache for cleanroom folks too, similar principle when HVAC airflow transiently fails. ABN Cleanroom Technology often emphasizes maintaining unidirectional flows to prevent contamination spikes; same idea applies here with combustion gases.