When silicone is soaked in pure diesel fuel it tends to swell up, but this a rather slow process though. It takes several hours in raw fuel.

In head a guess that the silicone is mainly exposed to oil and other fuel residues so it may well be fine for a long time. As noted it was not the seal that failed but rather the aluminium counter piston. So back to the drawing board and make a stronger counter piston, that might be all you need, plus a little more ether in the fuel...