It looks like newer versions of Chromium store Widevine in ~/.config, at least on Linux (no idea how things look on Windows/macOS).

On my machine, I have a ~/.config/chromium/WidevineCdm/latest-component-updated-widevine-cdm which contains:

{"Path":"/usr/lib/chromium/WidevineCdm"}

and Widevine is then stored in /usr/lib/chromium/WidevineCdm/_platform_specific/linux_x64/libwidevinecdm.so.

However, another user reports having this in the latest-component-updated-widevine-cdm:

{"Path":"/home/myuser/.config/chromium/WidevineCdm/4.10.1610.0"}   

and the actual library is at ~/.config/chromium/WidevineCdm/4.10.1610.0/_platform_specific/linux_x64/libwidevinecdm.so.

It'd probably make sense for QtWebEngine to read the path in that JSON file and also search Widevine there (in the proper subdirectory) in addition to the current paths.