The standard Sallen-Key design assumes you use perfect opamps.

An LM324 is pretty slow as opamps go, I'm surprised it shows the filter working as well as it does.

Perform a few more simulations, changing the opamp type you use. Use a faster opamp, a slower one, and a perfect one. I don't know LTSpice specifically, but most simulators have a generic opamp that you can set the parameters of, or failing that just a voltage source block that you can set a high gain on.

What's happening is the unmodelled increasing phase shift of the amplifier is changing the ideal response of the filter components.

It's not really a good idea to try to 'predistort' the design of the Sallen-Key to compensate for the amplifier speed out to the problem frequency at 1MHz where the response lifts. Firstly, with these component values and amplifiers, the passband and transition band are correct. Secondly, the bandwidth limit of opamps is not well controlled, so may be a bit different with every new build.

There are two ways to improve the response of the filter. The first is to use faster opamps. However, this only tends to move the problem up in frequency rather than eliminate it totally. Using faster opamps than you need also causes other problems. Slow opamps let you get away with bad layout or decoupling, fast opamps punish you with instability.

The second way to handle a bump in the stopband, if the continuing deep attenuation of the stopband is important to you, is to use a low order passive 'roofing' filter, in your case cutting off around 300kHz.