Why should dynamic range be lower when using ES?!
This is a question for someone with far more technical knowledge than I have, but I believe from past things I've read, the reason for it is that without the mechanical shutter closing after each exposure, stray light can impact the image following the exposure, introducing slightly more read noise. That's why you see the option for "Electronic First Curtain shutter," which still has a secondary mechanical shutter, and does not impact DR at all. That way, with the mechanical shutter closed, there is no additional light to generate read noise/heat on the sensor circuits for the brief milliseconds it takes to process the exposure.
Obviously there are ways to manage this, since Canon has improved the ES dynamic range in every release of a camera, but it's still slightly visible in the EOS R3 charts. It really doesn't make a difference at this point, though. Previously, Canon was also limiting Electronic shutter on the R5/R6 to 12-bit readout, but I'm sure with all the marketing around the uncompressed raw at 30 fps, they aren't limiting that anymore.