Validating part of the proof for both sat and unsat

[summer-yf]

Hi, I'm wondering if it's possible to validate parts of the DRAT proof, or only 1 of the clause from the proof, to see if it cause conflict.
First I tried deleting one of the clauses inside the DRAT proof for a UNSAT problem. It shows verified without running '-f', and NOT Verified if running with '-f' as option.

Then I tried adding 1.one clause (not deleted) from the proof to a SATISFIABLE problem, 2. The second time I tried adding its negation.
both shows:

c WARNING: RAT check on proof pivot failed : [63593] 869 6198 6718 7658 8954 0
c RAT check failed on all possible pivots
c failed at proof line 7647 (modulo deletion errors)

s NOT VERIFIED

Also I tried adding 1. one clause from the proof to a UNSAT problem. 2. its negation. Both shows the same except one has 15 clauses in core.

c parsing input formula with 13176 variables and 345426 clauses
c finished parsing
c start forward verification
c 16 of 345426 clauses in core
c 1 of 2 lemmas in core using 16 resolution steps
c 0 RAT lemmas in core; 0 redundant literals in core lemmas
c optimized proofs are not supported for forward checking
c VERIFIED derivation: all lemmas preserve satisfiability
c WARNING: no empty clause detected, this is not a refutation
s DERIVATION
c verification time: 0.295 seconds

Am I understanding anywhere incorrectly? Or is the function only available for full refutation check?
Thank you so much!