I’ve installed the latest 2.1.50 Qt5 Beta 2 on Manjaro VM and it seems like it doesn’t include that file in the appropriate directory. The directory hierarchy for the installation changed. platforminputcontexts folder is now located at: /usr/local/share/anki/lib/PyQt5/Qt5/plugins/platforminputcontexts/.
During investigation I’ve noticed that there was also another folder named platforminputcontext instead of platforminputcontexts located at /usr/local/share/anki/lib/PyQt5/Qt5/plugins/. This one contained libfcitx5platforminputcontextplugin.so. After I’ve cut and paste the file into platforminputcontexts folder fcitx5 started to work.
In conclusion it’s needed to fix the output folder for that plugin.