The documentation indicates that CUTOFF and OFFCUTOFF should differ for some ASEC records, at least for secondary individuals under 15 who are in the CUTOFF universe but not the OFFCUTOFF universe.
However, these variables are identical in the 2020 ASEC, and 0.2% of records have the 999999 NIU code for both CUTOFF and OFFCUTOFF (these are all under 15).
The Census Bureau updated their poverty universes in 2020, removing differences between official poverty variables (i.e, OFFCUTOFF) and non-official poverty variables (i.e. CUTOFF). As you can see in Table 1 on this page, the official poverty rate and the fixed poverty rate are the same. Thank you for pointing out this discrepancy; the CPS team is working to update universe statements in the documentation to reflect that the universes for CUTOFF and OFFCUTOFF are the same.