RDD.cache() called early creates problems

First issue I post here.  I noticed that when I call RDD.cache() early in my code, the results are all wrong!
If I remove the call to cache(), or I add cache later in the code, after the first map transformation, it works fine.
The graph is created from a data structure that already contains the random.

I have posted versions that work, and versions that don't work here in this gist.

https://gist.github.com/mitchi/edd9637687cf47fac2616bb72932f8e7

here is an output that works : 

Colors of the graph

3 2 1 3 2 1 1 4 2 3

and an output that doesn't work :

Colors of the graph

25 16 36 49 3 1 6 15 10 3