There are probably works somewhere as you guessed but I've not found anything official.
Check back in a week or two, the train is definitely not sold out.
I messaged the Hungarian railway company and they replied with the below:
”Dear Passenger,
Thank you for your inquiry, we would like to inform you:
Tickets are not available for this date yet, the sales have been temporarily blocked. The pre-booking period usually starts at 2 months before the journey. In exceptional cases, this period may be shorter due to technical or commercial constraints such as work of railway construction, changes of summer/winter schedule or special offers. The booking period could start at any time, the exact date unfortunately unknown.
Please try to book again later.”
The exact same thing happens to me but we will go from Budapest to Prague on October 16