Munich to Krakow Nightjet

There is a European wide train schedule rejuggle twice a year on the second weekend in June and December. I would expect you could see something in late September or October.

It's due to the timetable change. Here is what The Man in Seat 61 says about it:
Tip 1, the December timetable change
All European railways change their timetables at 24:00 on the 2nd Saturday in December.
Operators never maintain their usual booking horizons for dates after the timetable change.
This is the Big Annual Timetable Change. There's a minor change on the 2nd Saturday in June, but the December one is the Big One.
So you can easily look up trains on (say) 13 December, but if you try to look up trains on 14 December you'll find no trains show up until much closer to departure date. Or you'll find some trains are loaded but others aren't. Or trains are shown but won't book properly. Whatever. It's all to do with the Big Timetable Change. Got it? Just be patient!
French, German & Austrian railways (SNCF, DB, ÖBB) usually open bookings for dates after the mid-December timetable change in early to mid-October, so the normal 90 days shrinks to 60. And even then, the French only load data until the beginning of January, there's another delay before dates later in January are loaded.
Spanish, Portuguese, Hungarian, Polish, Slovakian, Czech & Bulgarian railways usually don't open bookings for dates after the mid-December timetable change until late November or even early December, so the 60 or 90 days shrinks to as little as 10.
For places such as Turkey or Lithuania, you're lucky if the timetable data is loaded the day before the new timetable starts.
But don't worry, no-one else can book either. The train won't sell out, you'll still see cheap tickets when booking opens. Just wait!!!

Ah! Thanks a lot!! This is so helpful! I was looking it up EVERY DAY, and it was getting frustrating! So good to know that I don't have to check until late-November or early-December.

for the sleeper, I'd try mid-October, as suggested, because you'll want to make sure you get the type of room you want.

Oh! Thank you for pointing that out! I think I focused on Poland, but not on Germany. The train leaves from Germany, so I should check early to mid-October. Thanks again! I will keep that in mind.

It isn't actually a Nightjet train, it is a Euronight train from Warsaw to Salzburg.

You may therefore find better prices through German Railways or Polish Railways, rather than through OeBB. Maybe look at the three websites for within the next month and get a feel for who offers the best prices.

Oh I see! You are right! I just checked all three websites and Deutsche Bahn appears to have the cheapest ticket. Here are the 3 websites I am checking. Please correct me if I am wrong:
Austria: Oebb (https://shop.oebbtickets.at/en/ticket)
Germany: Deutsche Bahn (https://int.bahn.de/en/)
Poland: PKP Intercity (https://ebilet.intercity.pl/)

They are the right websites. Conventional wisdom says that PKP (Polish Railways) should be the cheapest. I've just done an unscientific single date check for 8 September, and PKP are 40 Euro cheaper than DB (German Railways).
The drawback is that PKP only open bookings 30 days before travel, at least for day trains. AFAIK it's the same for night trains.
It's hard to know whether 26 December will be deathly quiet or very busy on the trains, it'll be one extreme or the other.
Looking at the train consist on the vagonweb website it is a single sleeping car, two couchette cars and a seating car, so sleeper capacity is fairly limited.

True! That's what worries me the most. Fewer sleeper seats and the date being Dec 26. Good to know I have the right websites. I may book from either DB or OeBB in October and then see if I can cancel this for a cheaper PKP ticket in November.

I am impressed. Under 12 hours, and under $600 for a couple in a 2 bed compartment. Good for a NightJet.