Long time fan here
I will do so as soon as possible (I only have internet access in my mobile right now, though I'll see if I can use it as a hotspot).
In addition: I suspect it has something to do with Indesign's font subsetting (at least my googling suggested as much).
Update: Just tried replacing the subsetted file with the original file. Problem solved... But I still want to know what is it that Calibre interpretes as "this font must be slanted".
I've found this to be a somewhat common issue with embedded fonts in Sigil and Calibre.
I'll upload the font, the subsetted font and the subsetted exported font ASAP.