Really a few guesses of sorts...
1. Possible roof leak or leaks. The outer membrane of the roof gets permeated, the water has no where to go... except thru cracks and crevices of the foam.

2. Differences in temperature of the roof and attic. In a humid climate this can in various times mimic a roof leak as the insulation sweats or the humidity in the air condenses somewhere between the roof membrane and the spray foam. Probably happen most in a spray foam application gone wrong where there are air leaks in the spray foam membrane.

3. Poor application, too much spay foam applied at once. Too thick, too humid, too hot during the application process. So many variables...

Given your picture I probably lean more towards 1 or 2, but #3 could play a role as well.

I am NOT a spray foam specialist... this is opinion only. (I have been in alot of homes over the past 26 years.)

I am AC & Heating Service, Repair & Retro Fit installation serving Katy, Texas area.

This could be an issue with the application of the foam, as typically foam should last as long as the house does. Given that the discoloration is sporadically popping up though, it may be issues 1 or 2 that Austin Air outlined.

Twintech Heating & Cooling

https://twintechheating.ca/