There is a bug in 18.08.01 regarding rather erratic text behaviour. I wonder if that could be your problem? See:
https://forum.shotcut.org/t/text-filter-presets/6644
It’s fixed for the next release (18.09) but in the meantime you may like to go back to an earlier version (if you are using 18.08 that is), and see if it works OK.