Yesterday, I was taking a flight on an EMB-175 from EWR. There were two unbooked FC seats and I was first on the SB list. The GA didn't clear the list before starting pre-boarding. When she called 1K's, I walked up to the BP reader and asked the GA if she was planning to clear the waitlist. She confirmed I was first on the list and told me to go ahead and scan my Y BP and then take one of the open FC seats. I did, and kept refreshing the upgrade list on the app until finally I saw my name clear about 10 minutes later.
Theoretically, someone could have done a TOD purchase of that seat before the FA got to clearing the list, which would have made things interesting. I'm with the others who say the list should be cleared at least 5 minutes before pre-boarding.