It's the third time I lost progress now. Continuing my game from the main menu sometimes rolls back to multiple hours of progress before I last stopped playing.
As we can't save manually, I'm now never sure if the game is really saving or not.
From what I was able to observe, I wonder if it's not related to the Quick Resume function of the Series X. IIRC, each periods of gameplay I lost were the one I made after a Quick Resume of the game (instead of stopping / restarting the game entirely). In each cases, I think I got a message saying the game could not connect to the UbiSoft server (or something similar).
It seems appalling that the single player mode requires a constant connection to your servers in order to save the game and that any connection drop lays waste to the entire saving mechanism.