U2F with Nitrokey FIDO2 and different Browsers (Firefox / Vivaldi)

I used to login to github with Firefox and U2F. Now I switched the browser to Vivaldi. The U2F login didn’t work. But after de-register and re-register the key on github (with Vivaldi), it worked also with the new browser. Has anyone had similar experiences? Why is that?

(Both browsers in the latest version on Arch Linux)