I ended up guessing after no one found anything, and later used an online solver (https://mrgris.com/projects/minesweepr/) to prove that there was in fact no safe square. BTW this solver is pretty cool (and different from the one I linked in the other thread), it’s more of a thing to play with and learn from than a tool. The links to use it (live demo / board solver) are on the right side of the website. Check it out, even if just for a fiddle!

Edit: I did have to input the whole thing manually, by which I mean just a cropped part that includes the unknown territory.

Oh, and I think the game bugged out. It does sometimes fail to generate a valid solvable board but I don’t think it happened that time because it would show a toast message and I don’t think I missed it. It didn’t show up in the toast history either.

Shortly after this encounter I figured out that the generator has quirks for large boards, making a pattern. If you look at the fully solved board (embedded elsewhere in this comments section) you’ll notice that the outer edge of the board is always safe, and the left side of the screenshot has two full columns of safe squares. This turns out to be a repeating pattern (not of the mines, just the safe columns). The end result is that it’s no longer fun to play, so I turned off solvable boards and I’m now playing without. That’s how I got to today’s post.

I think the generator incorrectly expected me to have the left edge. Or something weird like that. I played this game a lot and this was the only time I encountered something unsolvable when it should have been solvable.