I’d think that you’ve probably had a less than perfect seal and have been drawing oil into the bottom of the ports from the valley area.
FWIW, I’ve used Felpro 1213 S3 in the past and probably placed too much faith in the PrintoSeal beads doing their job on my application. While the 1213 S3 gaskets are a little thicker than the regular (non steel shim) 1213 I still needed something thicker. I’m currently using .090” Cometic intake gaskets on my 410 and have had good success with some blue Hylomar (it’s fuel resistant) around the intake ports and grey Permatex around the water ports and on China walls.