In regards to Stadia, it seems to be performance related issues this time around. Due to the fact of being aware of performance issues on Stadia, I've got reason to believe that contrary to the alleged Google claims, they have been sent various iterations of the patch to be tested and evaluated enough to come to the conclusion that it needs more time to optimize for Stadia.
Taking the Stadia version as an example for Patch 6, we felt that performance wasn’t as desired, so we decided to hold it back to work on it for longer.
Source:
Hotfix #16 Update PostI can't comment on Larian nor Stadia's position, but I can refer to a scenario during Divinity Original Sin 2, where GOG users were delayed a particular update due to some issues with GOG pushing the update to their servers. I don't think that's the case for Stadia and patch 6, but since Stadia is a closed cloud server system under Google, I can only imagine the logistics and back & forth that may take time just due to that, along with actually working out the performance issues on Stadia hardware.
That's all we can really guess at this point, and I find it important to make it clear that I'm not employed by Larian and thus this is more a comment from me being very active in the community and informed enough to have some opinions on it, rather than any official response to hold Larian to

Fingers crossed that it doesn't take
too long to sort out the performance issues though
