Hi,
In Spanish we do not use ligatures. However, some fonts always show ligatures, such as ae, and it is impossible to deactivate them. I have noticed this behavior in the Gillius and Baskervald otf, in example. I use Scribus 1.5.6.svn in Kubuntu 19.10. I do not know if it is a problem of Scribus, Plasma or Kubuntu.
I appreciate any help.