They pushed out fixes on 7-Feb-2022 to address the high volume of stale gridpoint forecasts that I'd alerted them to back in October, 2021.
The good news is that it seems to be working. My tracking of failed / stale requests has only returned 2 each on the last couple of days. So advforecast2.php V5.18 is working just fine.
I want to give it a good week before I respond to the trouble ticket as 'likely fixed'.
They are still having an off-by-one on gridpoint coordinates for a point lookup, but that's at least nearby if not for the exact point requested.
