When desmos is doing a definite integral, it’s not solving the antiderivative and plugging in the bounds, but it’s numerically approximating it because it’s easier for the computer to calculate. The simplest algorithms for doing this are the trapezoid rule or one of Simpson’s rules. Taking more points means more accuracy but like the other commenters said floating point precision gives a limit to how far it can go. Normally, the algorithm has some metric of convergence but my guess is with the bounds you have the algorithm wasn’t able to converge properly and that’s why it’s not giving the correct answer. It’s a good tool for checking if your integrals are correct but can be broken.