"Goodnight Irene". "So Long, It's Been to Know Ya". "You Are My Sunshine".
All have entered into the folk process. They have been adopted by people who have no idea that they were written in the last century and that their composers are known. When people sing them they sing them as songs they learned from somebody else; a parent, a sibling, a friend, from a recording. They may remember the lyrics or the tune " "incorrectly" or add their own variations.
Frankie Armstrong told me she met a couple of German girls travelling in Wales who were singing Ewan MacColl's "Moving On Song". Unlikely they learned it from the original Radio Ballad. It had become part of their personal tradition.
Many of Ewan's songs have entered the tradition. "Shoals of Herring" is popular in Ireland and there are people there who will swear they learned it from their grandfather.
It doesn't matter the provenance of a song. If people adopt it as their own it becomes "folk".
Big Bill Broonzy once said, "If folks sing it it's folk music."