How do underwriters treat the rehab invoice trail when there is no seasoning yet?

Looked at from the lending side rather than the owner's, the cost basis question keeps coming up. On a delayed financing setup the file is supposed to document the purchase and the documented cost of improvements, and the loan gets sized off that basis rather than off appraised ARV. That works fine when the borrower used one general contractor and has a signed contract plus lien releases. It falls apart when the borrower self-managed, paying six subs and putting materials on a personal card, with some of the demo done by hand. So: how much of a mixed invoice trail do underwriters actually accept as basis, and does labor the borrower performed himself ever count? And separately, when the file switches from basis to appraised value after the seasoning window, is the same appraiser's ARV opinion honored, or does the refinance appraisal typically come in under the original as-repaired number because the comps in these neighborhoods are all unrehabbed? Better to know where the file breaks before anyone is holding it.