Sounds like the system was not looking at the OP’s status and was assigning a group based on being Basic Economy.

A few months back I had a few random
oddities on F tickets. Instead of group 1, I was given group 2 and 4 on a few random segments over the course of a few reservations. Even on multi segment travel days it would just be a single random segment misassigned. I boarded with F and was not challenged.