I haven't investigated this myself yet, but various users reported that Widevine stopped working for them after upgrading from Chrome 61.0.3163.100 to 62.0.3202.62.
Situation on Archlinux:
- Chromium: has /usr/lib/chromium/libwidevinecdmadapter.so, updated to 62
- qt5-webengine-widevine (AUR): has /usr/lib/qt/plugins/ppapi/libwidevinecdm.so and /usr/lib/qt/plugins/ppapi/libwidevinecdmadapter.so, still at 61
- chromium-widevine (AUR): has /usr/lib/chromium/libwidevinecdm.so, at 126.96.36.1998 which I think corresponds to 61
Situation on Gentoo:
- www-plugins/chrome-binary-plugins: has /usr/lib64/chromium/libwidevinecdm.so and /usr/lib64/chromium/libwidevinecdmadapter.so plus symlinks with chromium-browser, updated to 62
No matter how/if this gets fixed, I think QtWebEngine should probably prefer the /usr/lib/qt/plugins/ppapi/ path over Chromium's so an older version can be dropped in easily in situations like this?