I seem to be having a problem with some of the PBR textures not changing with the season. I have the snow level set to zero. I double checked to see if the textures weren't seasonal, but they say they are.
Try setting the snow level to -2000 and changing the calendar month to a winter month.
It also depends where the route is located (ie nothern/southern hemisphere) as where the notharn hemisphere is in winter the southern will be in summer and vice versa.
The easiest way is to open up a PBR with "seasonal" in its name (but excluding those with "non seasonal"). I found 196 of them using Content Manager listed as Built-in, Packaged, Installed from DLS or Available for Download.
The first installed one that I opened up for examination had the correct texture-variant and season-selector tag containers in its config.txt file but there was no difference in the listed texture names for the "0" and "1" season variants - only two seasons were set. So, despite the name "seasonal", it would not show any seasonal differences.
The next one I opened up had all 4 seasons listed ("0" to "3") with different textures for each. So it would show different seasons - according to your set hemisphere and calendar date (as pointed out by @hiawathamr above).
