I wouldn't say that the resolution is worse... but rather that the codec compresses the image so much that the end result looks more like a much lower resolution than it should have.
And that was obvious from the get-go, since the low internet speeds posted as necessary for 1080p60 and 4k made it very clear it would be hyper-compressed compared to industry standards.
Technically the resolution is worse on both games. From what I understand, XB1 X runs both games at native 4k. Stadia by comparison runs RDR2 at 1440p on a 4k stream, upscaled to 4k, while, Destiny 2 is apparently 1080p on a 4K stream, upscaled to 4k. Add to the lower resolution the compression artifacts from the codec, and it is clear to see why the screenshots look so much blurrier on Stadia.