The temperature of the seal has an effect on its kinetic (sliding) friction; the ‘hotter’ it is, the greater its diameter, the greater the friction. I measured the kinetic friction of a polyurethane closed face (more like HW than Air Arms) seal in a 25mm cylinder at -7C (1.4N), 20C (4.38N) and 44C (7.36N). That suggests a huge rise in the drag on the piston when the seal becomes hotter, but that’s only part of the picture, because the seal also expands in diameter (and to a far greater extent) as the air pressure in the cylinder rises during the compression stroke.

I repeated the kinetic friction test with the equivalent pressure on the seal face of 150 psi of air pressure, and the results were -7C (88N), 20C (111N) and 44C (115N). As the cylinder air pressure rises, the effect of elevated seal temperature on overall kinetic friction diminishes.

I doubt this explains the OP’s loss of muzzle energy; if it did, then every springer with a synthetic seal would have behaved the same on Saturday, and the forums would be awash with posts on the subject. I think a strong contender is evaporative losses from the cylinder lubricant due to the high temperature leaving a very thick, almost dry film, and other tests have suggested that piston seal kinetic friction is more than doubled with the loss of ‘wet’ lubrication. That would really bring the muzzle energy down.