Those were probably Greek Rite Catholics, which is also popular in the Rusyn ethnicity (a Slavic group in the areas between Poland, Ukraine and Slovakia.) Different history than Polish Catholics.

https://en.m.wikipedia.org/wiki/Religion_in_Ukraine#Demograp...

https://en.m.wikipedia.org/wiki/Rusyns

https://en.m.wikipedia.org/wiki/Eastern_Catholic_Churches