I saw this first in Lyx (see https://www.lyx.org/trac/ticket/11233), but discovers this happens in other Qt apps and only under Plasma. Under IceWM it was working correctly. If in the properties of a font the truetype name is set to "Book" real italic is used (e.g Times New Roman), if it is set to "Regular" fake italics is used (e.g. Tinos). I have seen this in lyx, sigil, calligra words. This is only happening in workspace area, In the UI italic is working correclty (in systemsettings > fonts > fonts). It is also working correctly in kate.
On openSUSE Leap 15.2 I do not see this anymore. Did some testing and could not find a font that used fake italics. I assume this means this bug can be closed.