Hi All,
I wondered the exact same thing and hope I didn’t screw up. I got the ‘Game Pass for PC’ for $1 for the first month as a kind of trial of MSFS, decided I did want to own it, and then bought Premium Deluxe from the MS store for $95.99 (20% off for Game Pass subscribers). It’s true I haven’t yet cancelled my Game Pass, but from everything I’m reading, when I do, I’ll still have access to the full MSFS Premium Deluxe.
MS is a bit unclear on this exact point, but they do offer the Game Pass discount as a way to ‘own’ the game. It would be very odd for them to do that if owning the game also required being a Game Pass member. Why buy it then (at least while it’s still on the Game Pass)? You may think ‘because I want the Premium Deluxe content and that’s not available on the Game Pass version of MSFS’. However, note they also offer that same 20% discount for the Standard Edition (it’s $47.99 with the pass). In none of their language do they clarify that when they say you’ll ‘own’ the game, that you must also remain a member to access it.
It seems some people that had problems may have bought an upgrade ONLY to the base, standard game that’s part of the Game Pass (maybe through the in-game marketplace?). In that case, it makes sense that they’d lose access to the game – the upgrade is a kind of DLC on top of the base. What MS is selling in the store (outside the game) as the MSFS 2020 Premium Deluxe is the complete game and I expect (really hope 
) that I’ll keep that once I cancel my pass.
Further reading about this exact issue:
Good luck (for you and me )!
Justin