The Interrail app runs offline based on a downloaded data set so is not always 100% accurate.
It is better to use the websites or apps of the train operators or the likes of DB and OBB who have accurate data for much of Europe
https://www.bahn.com/en
https://tickets.oebb.at/en/ticket
Looking at this, it appears that there is disruption on that route, the trains towards Prague over the next few weeks are only running from Szob.
The planner gives a departure from Budapest-Nyugati at 11.08 to Nagymaros on regional service R 2324 with a rail replacement bus for about 20 minutes from there to Szob, with your booked train resuming it’s normal times from there.
Your res will be valid on the same train, the connecting train and replacemnt bus are not reservable.
There is an alternative from Budapest-Keleti at 11.30 with a change in Vienna.