in the "compositing" section on your layer, have you switched the Scale Filter setting to "Linear"?
This looks like what happens when you scale down a large sprite with the "Nearest Pixel" scale filter.
Yeah, I tried setting the filter to linear in compositing... that fixed the issue. But now there's another problem: a white line around the sprite due to the alpha mode. Without premultiplied alpha, it gets even thicker. And yes, I tried both scaling down the sprite manually and using a version pre-saved at the target resolution.
https://imgur.com/a/otjfv41