Eric: Obviously not a Grant, but one of my farm implement ejector Smith guns fired on closing. This is David Williamson's image and upper edge of the sear, after thousands of shots (and usually the right), can become rounded.
As you observed, if the barrels are not fully opened, the sear is not fully seated in the notch. Smiths often need an extra effort at the end while opening the barrels, after the ejectors have tripped, to fully cock the hammers.
The cure is a very good gunsmith with a small file
and giving the barrels that extra push.