There's a bug in Illustrator (all versions MacOS 10.12.6):
when there's a non-vectorised text mask in a file and you do select all -> vectorise text, the non-vectorised text mask will not be vectorised.
Then when you re-open the file on a computer without the active fonts, it won't give you a warning that there's a missing font!
Only when you click on the mask it will say that there's a font missing.