Well, you're playing in Snapshot, so there will be bugs and glitches here and there. You're close enough to see that thing if it would be there, as it's so big, that you just can't miss it. It probably just didn't spawn where it was supposed to spawn. Tough luck.
Could you maybe give the seed, so I (or someone else) can check out if it would really spawn there, or is it just glitched/bugged map. I can't check it out now, as I'm not home, but I could check it out when I get home.
I have had a few glitches with mansions happen recently. My first map went off without a hitch. My 2nd one I gave up on because I was making zero progress it seemed. I went to village closer to my original spawn point thinking maybe if i got a map there it might be closer (just a hope). Well I bred a new cartographer and purchased a new map and was surprised to find it was a carbon copy of the one I gave up one. I considered this fate so it took a few days of play time but I finally went west far enough to find the lattitude and then went due north until the tiny lil dot turned into a large dot and started to move on the mansion map. My first concern was that although the dot was moving, it wasnt filling in the map details (colouring it in). I reached the destination which was a wooded area, but no mansion. i checked both maps, who have different numbers, both showed me being at the right spot, but no mansion. i'm now wondering if mansions are permanently glithced on this world for me. For information I use no mods and play in survival mode.