Wecome to the forum Xaphedo.
I've just done a quick check - on WIndows 10 with Scribus 1.4.6 - on a document containing three text frames, formatted in:
* Roboto Medium - True Type - Embeddable;
* Dosis Medium - True Type - Embeddable;
* Open Sans Regular - True Type - Editable.
Exporting to PDF, the dialog I get after pressing the "Embed All" button is shown in CaptureExport.PNG
Both Roboto Medium and Dosis Medium are set to be outlined which seems odd because they should be embeddable.
(See here
http://www.adobe.com/uk/products/type/font-licensing/font-embedding-permissions.html for more information.)
After export (Document-1.pdf) and reading in Acrobat Reader DC I get the properties seen in CaptureAcrobat1.PNG
One of the fonts - probably Dosis Medium - seems to have been converted to a Type 3 (Postscript) font, which doesn't sound right to me (see below).
The Roboto Medium text cannot be selected but the Dosis Medium and Open Sans Regular text can be selectable.
Going back to the document and the export dialog I pressed the Embed All button again, then selected Dosis Medium, pressed the "Up Arrow" (to move the font from Outlined to Embedded) and then selected Roboto Medium and pressed the "Up Arrow" to end up with the dialog as shown in CaptureExportMissing.PNG. One font to embed, no fonts to outline, which seems wrong to me.
[Forum restrictions mean that I have to split attachments on multiple posts from this point.]
After export (Document-1a.pdf) and reading in Acrobat Reader DC I get the properties seen in CaptureAcrobat2.PNG
Now I have the situation where Dosis Medium and Open Sans Regular seem to be embedded properly - but note that Open Sans is missing the Actual Font information - but there's still no Roboto.
Doing a similar thing as above but also removing Open Sans Regular from the list of embedded fonts - CaptureExportNoFonts.PNG - gives me Document-1c.pdf and CaptureAcrobat3.PNG. Something strange is happening here.
I have a feeling that Scribus isn't showing users the full story of what it's doing with fonts.
Because of this it's difficult to diagnose what's going on with the Roboto font. It's supposed to be embeddable but Scribus isn't embedding it for some reason and it's not telling the user why.
Maybe one of the font experts in the forum can help more.