Version 5 of qtwebengine does not search for widevine in the paths where Gentoo's www-plugins/chrome-binary-plugins installs it. The correct path was only added in version 6, it should be backported, see URL for the commit. Note that the audio codecs part of the commit does not apply cleanly. I didn't bother to fix it, just kept the path changes only. Reproducible: Always
fwiw qutebrowser's ebuild been handling this by passing: --widevine-path="${EPREFIX}"/usr/$(get_libdir)/chromium-browser/WidevineCdm/_platform_specific/linux_x64/libwidevinecdm.so to qtwebengine when USE=widevine, but still passing it for qt6 webengine (despite it can find it) because of EPREFIX and libdir (albeit libdir should be static given chrome-binary-plugins is a prebuilt and only available for amd64). Note both webengines (incl qt5) can find it in the in /opt if google-chrome is installed.
Created attachment 845519 [details, diff] Patch based on upstream commit that applies cleanly
Thanks for your contribution, this patch was added already with qtwebengine-5.15.8_p20230106.