I have a score printed as a pdf from Sibelius with all used fonts embedded but when imported into mobilesheets the Plantin font is not rendered.
(see attached pdf score as output from Sibelius))
It seems to mostly render correct using the "Quality" renderer in MobileSheetsPro with my latest code. As Snard has mentioned, the time signatures are rendering incorrectly. I switched to the "Speed" renderer and it actually all shows up correctly. I'm going to be releasing version 1.3.1 very soon (hopefully today) so you will be able to test this yourself.
Interestingly, despite having Sibelius on my PC, so I should have the fonts, it displays incorrectly for me. Looking at the font properties it says
OpusStd - actual font Foxit Sans MM
PlantinMTStd-Italic - actual font Foxit Sans MM
PlantinMTStd-Regular - actual font Foxit Sans MM
TimesNewRomanPSMT - actual font Times New Roman
Looking at a PDF that I have generated from Sibelius the font properties say things like
FPRLIQ+Opus Text (Embedded Subset)
So I'm not convinced you really have embedded the fonts. Maybe you need to check some settings somewhere.