Both available international ticket salesmen in Zagreb said the RAILPLUS is not valid in Bosnia.
Yep, sorry we didn't check it earlier. Somehow just thought that like every European company participates (some just for <26 and >60), and didn't even cared to recheck. Turns out that countries like UK or Ukraine are in, but BiH, Turkey and Albania are out )))
Ticket officer in Mostar refused to sell a ticket to Osijek, saying he can do it only for a direct train (strange).
Sometimes such things happen on small stations where they don't have access to the system or even good enough tariff tables to calculate the fare. But more likely your cashier just didn't bother to check tariff table (they definitely must have one in Mostar) or at least call Sarajevo to ask them.
In Beli Manastir the Railplus discount was again not available due to the fixed-price (ca 44 hrk) Edmonson ticket (!!!) to Pecs.
Wow... that's something. Did you happen to keep it? It would be interesting to see it