Grapheneos follows a slightly different upgrade schedule.
Previously google carried the pixel drivers in the AOSP updates (the OS on which Grapheneos is based) but a couple of months ago they removed them and this has made the updates a bit delayed, but they have always taken a couple of days to update. This time, the OS updates have taken a little longer than expected, but the security updates have been almost up to date.
Hopefully, thanks to the work they do, they won't need to delay updates much longer.