I'm on macOS and I had a similar problem where for some pages It didn't work and I had the following message
[5545:259:0319/215947.620131:ERROR:thumbnails_api.cc(280)] Failed to capture https://techcrunch.com/2025/03/19/aws-generative-ai-exec-leaves-to-launch-startup/
that's launching Vivaldi as a regualr App
(without the --disable-features=PrintCompositorLPAC argument)
I then saw that if I first scroll the page I want to copy to the bottom (example with the techcrunch page), then try to capture the page. it works properly.
I guess there are some javascript events linked to scrolling that updates the DOM and interfere with the capture action.