Please build Sonnet RPMs with libvoikko-devel installed. Now it seems you are
building Sonnet RPMs without Voikko support:

$ ls /usr/lib64/qt5/plugins/kf5/sonnet/
sonnet_hunspell.so

vs. build when libvoikko-devel has been installed when building Sonnet:

$ ls /usr/lib64/qt5/plugins/kf5/sonnet/
sonnet_hunspell.so  sonnet_voikko.so

Voikko library is needed for Finnish spell checking and now Sonnet needs to be
rebuilt if user wants to add Finnish spell checking in KDE or its side apps.

Or if it's not practical to include voikko plugin by default into standard
sonnet packages, then could you please create a side/plugin package which
install the sonnet_voikko.so library?