Thanks for providing an update, and sorry to hear that didn’t work. I’ve since tried to replicate the same issue on a WordPress.com site although I’ve been unsuccessful.
If you can share your experience in the form of a video using a service such as recordit or loom we can see exactly what’s occurring. Another method to record is from your phone and provide a link to the video after making it available on Google drive.
Alternatively if you’re unable to provide a recording please share at what stage do you encounter the “‘redirect_uri’ mismatch” notice? Is it after you press on the “Allow” button during setup, when Site Kit requests permission? If you want to share the full URL in your address bar here it may provide some insights into whats occurring.
I’ll also check your site setup from the Site Kit authentication service.
I’m still waiting for this issue to be resolved on my site.
We provide support during the weekdays only, hence the late response. Hopefully we can determine the root cause with the above information. If you have any questions feel free to ask here.