Did it tell you why? If it surrendered the lock to a competing process, it should report that as previous INFO messages. If it doesn't give one of those, then it probably just thinks there are some tuples it can't remove yet somewhere. What did it give earlier up in the verbose output, for the number of removed and nonremovable tuples?
I just had an extra look at that, and I just got trapped a0f5954a that bumped max_wal_size from 128GB to 1GB.. Sorry for the noise.