I'm not sure where to put this, because I don't think it's a bug. Just a bit of confusing and missing text.

When I look at the Pilgrims in the 'Population Details' segment of the Senate screen, they, like all other population affinities, have their own set of bonuses and penalties when it comes to ideology. However, their population traits are odd.

They have Materialist - bonus science from science events. Makes sense, I like it.

They have Spiritual - bonus religion from religious events. Also fits them and the dual nature of scientist-mystics is good to see here.

They have Curious - bonus science from pacifist events. Also fits their themes of exploration and discovery rather than war.

So far, so good. Nothing wrong here and it looks exactly like the Pilgrims to me.

Here comes the odd one -

They have (and I'm guessing here) Relativist - which muddles everything up. It appears to override Materialist, halving the amount of science generated from science events, and adding that same amount to the ecologist ideology.

The description is all garbled with text: %PopulationPoliticalTraitRelativistTitle - which I assume means that it was a line in the xml files that should have gone somewhere else, and was accidentally added or left in the Pilgrim description on release.

Finally, unlike all the other population affinities that I've seen, the Pilgrims lack a description of who they are. It just says "TBD" (to be determined).

So, the Pilgrim minor faction got missed during the last round of clean-up before Update 1 was released. It would be nice if Amplitude could take the time to fix it soon, not only because it jars with the aesthetics and overall high quality of the game's presentation, but also because it has a negative effect on the Pilgrim population affinity. 

The Z'vali also have a TBD description.

Maybe file a bug report? https://www.games2gether.com/endless-space-2/forum/114-bug-reports