You mean when you measure your tire after a race it is physically smaller than what you measured before the race started? Then you must be leaking air somehow. During the race the tire heats up, which makes it more pliable - and also the air pressure in it goes up, so air can leak more easily.
I suspect a bad valve stem, and it's a cheap easy part to replace to see if that cures the problem. Although first I'd use a valve stem core tool to make sure that the valve core is fully seated but not stripped, and I'd pull the core out to make sure that the little O-ring on it looks good. We once had a leaky core because we assumed that all cores are the same, and hadn't paid attention to swapping them between different brand wheels. And of course you'd want to check for dirt that might have gotten into the valve stem while pulling the core out and replacing it. Look for cuts or even weak places on the valve stem itself - racing contact can leave small cuts and abrasions. We've had them crack where the stem meets the part that seals against the wheel - both at the top of that seal where the stem ends, and also its bottom side that contacts the wheel.
I'd also carefully look for dings in the tire bead seating area all around the rim (inside seat too). If you decide to change the valve stem then I'd clean the bead seats and look for pits from rust. Finally, if you're running a bead lock on that wheel I'd make sure that it and its mating surface are flat and that all of the bolts are torqued correctly.
You could have a small hole somewhere in the tire, or around a plug you've put in. Any sidewall blemishes due to racing contact are suspect. The old spray it with soapy water trick might find it, if you over-inflate the tire by about 10 to 15 pounds (or a little more than what you normally expect to pick up during a race). That much over-inflation should not permanently grow your tire as long as you're only keeping it that high for the few minutes it takes for spraying it to reveal a leak.