PNR monitoring keeps its attention on travel that is still ahead of you. A leg that has already departed stays out of the pending changes list, so the PNRs waiting for your review are the ones with something to act on.
Cancellations and changes on flights that have not departed still come through, including at airports where the timezone data is incomplete.
When you turn on monitoring for a PNR, Yaalago uses the PCC the PNR came from, and asks you to pick one only when no matching credentials are on file.