This need for updating fonts is related to emoji keyboard I have created for Jolla. As "the app" it is just QML files for keyboard I am not sure about this font loading.

I have tested with fc-cache and actually I found only "fc-cache -r" to be only reliable solution to get font updated even when I reboot device. More mild option "fc-cache -f" didn't to do its job in some very minor font updates. I do not know why.