There’s no train after Zagreb towards Serbia and Bulgaria due to engineering works and political issues. The Interrail map is totally outdated and wrong unfortunately.
The route would be Zagreb - Budapest + night train to Bucharest, also possible to get off in Craiova and connect to Sofia. Don’t expect fast trains, take your time :)
If you’d like to have an adventure:
Zagreb to Beograd
06.31 from Zagreb to Novska, change there to Tovarnik (arrives 13.25)
need to walk into Serbia (about 2 hours)
14.58 / 17.18 / 18.53 from Šid to Beograd (arrives 16.29 / 18.53 / 21.25)
Beograd to Sofia
03.54 / 07.25 / 13.01 / 15.06 from Beograd to Niš (arrives 10.37 / 13.42 / 19.20 / 21.34)
from Niš to Sofia by bus (about 4 hours / 15 €).
Sofia to Istanbul
18.45 from Sofia to Halkalı (arrives 06.34 or later)
from Halkalı into Istanbul by frequent suburban train
But of course every normal traveller will go via Bucuresti or Craiova to Istanbul...