Thanks for letting me know the logic here. My algorithm has changed since I posted this, so I don't get these 'lucky guess' errors any more

You got lucky.

(additional part of the map removed for clarity)

      x        
1 1 2 2 2      
+ . 1 + .      
1 1 1 1 1  

and

      x        
1 1 2 2 2      
. + 1 . +      
1 1 1 1 1      

Both of there solutions are possible and you made a lucky guess instead of returning '?'

Thought I would post this here for anyone to check out:

Test #23: Your solution succeeded where the original didn't.
Might be that yours is smarter... Or you made a wrong (but successful) guess somewhere ?
Please post a comment in the discourse about that, providing the information below:
 
FAILED TEST !
 
Number of mines: 9
Original map (not necessarily the expected one !):
0 0 1 1 1
0 0 1 x 1
1 1 2 2 2
x 1 1 x 1
1 1 1 1 1
1 1 1 0 0
1 x 1 1 1
1 1 2 2 x
0 0 1 x 2
0 0 1 1 1
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
1 1 1 0 0
1 x 1 0 0
1 2 2 1 0
1 2 x 1 0
1 x 2 1 0
 
Game map:
0 0 ? ? ?
0 0 ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? 0 0
? ? ? ? ?
? ? ? ? ?
0 0 ? ? ?
0 0 ? ? ?
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
? ? ? 0 0
? ? ? 0 0
? ? ? ? 0
? ? ? ? 0
? ? ? ? 0
 
Your answer:
0 0 1 1 1
0 0 1 x 1
1 1 2 2 2
x 1 1 x 1
1 1 1 1 1
1 1 1 0 0
1 x 1 1 1
1 1 2 2 x
0 0 1 x 2
0 0 1 1 1
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
1 1 1 0 0
1 x 1 0 0
1 2 2 1 0
1 2 x 1 0
1 x 2 1 0

Expected:
?

I have the same problem - able to pass 100,000 big numbers and not the lower random tests. I noticed that the signs are the same for those tests, whereas the signs are always - and + for the largest random test. Any hints on how to solve the off-by-one errors?

I added those tests now also in final tests. I don't see what makes these tests so special that you would not pass all of those as well.

Was able to submit a solution without passing all of the 3 initial toy tests. Was this intentional?

you can print or log the puzzle layout, just code it into an array/list

the random tests do not test for some possible winning positions, was able to submit a faulty solution