-If you can get at the logs, you might find an entry in them detailing why it's dropping the mod when you apply it.
c:\Users\*yourname*\AppData\Local\Introversion\Prison Architect\debug.txt

-If you have a second machine around, I'd suggest running a test on that..

ps. It is probably something on a lower level blocking regular operation on your current station..

Also please verify if Christmas Mod Mashup is working at your end or not..
-To help me track if this is happening to similar mods two, or if it is isolated to {Winter Snow}..!

There was nothing about snow in the debug txt. but it did have some problems with the extra crimes addon that you made.

The christmas mod mashup also works fine.

Very weird.. I'd expect Xmas to show the same non conformity..!
It then does point to something in Winter Snow getting scrambled somehow.. and not a problem with your game's installation.

Maybe you could check what's inside the mods folder? c:\Users\*yourname*\AppData\Local\Introversion\Prison Architect\\Mods\617485081\ should have the following:
-manifest.txt
-thumbnail.png
-data subfolder with;
-objects.png
-orgchart.png
-papersheet.bmp
-people.png
-texture_corkboard.png
-tileset.png
-ui.png

Check if they are there and if you can open all, to rule out bit-errors.. or trouble downloading!

They are all there and all openable

Just tryed again... and got another flat ♥♥♥♥♥♥♥ no