PFS Page corruption issue and question
See https://www.sqlskills.com/blogs/paul/pfs-corruption-after-upgrading-from-sql-server-2014/
If this database was upgraded some time from SQL Server 2014 or earlier, you may have had the problem before the upgrade, but it was undetected by CHECKDB.
You should contact Microsoft Support, which will probably tell you to run a DBCC CHECKDB (or CHECKALLOC) with REPAIR_ALLOW_DATA_LOSS (if that's the only error, it should not cause data loss; it would only fix the PFS type). You may need to run that twice to fix it; after the first run, you may still have an error such as:
Page (1:111) in database ID 11 is allocated in the SGAM (1:222) and PFS (1:333), but was not allocated in any IAM. PFS flags 'MIXED_EXT ALLOCATED   0_PCT_FULL'.