Two reproduced formula defects in loan-calculator 1.2.2 prevent IOF gross-up from reconstructing the requested net principal. Progressive Price pairs tax weights with the wrong amortization shares; constant amortization adds an extra discount-sum divisor. An exact synthetic constant-amortization example requests 1021: the library returns 1024 gross, its own deductions total 6, and the recovered net is 1018. An independent cash-flow reference and 36 cases give 14 / 0 / 14 failures for the original / proposed correction / restored original; all 12 regressive controls pass. The archive contains the MIT-licensed release wheel, portable offline Python verifier, original results, proposed patch and SHA-256 checksums. Text is CC BY 4.0; original verification code and upstream code retain their MIT licenses. Official developer report: https://github.com/yanomateus/loan-calculator/issues/15Full article: https://www.gero.uz/research/articles/loan-calculator-iof-grossup-net-reconstruction.html Reproduced locally; maintainer review pending. No accepted fix, current tax-law correctness, deployed customer loss, security impact, bounty or worldwide priority is claimed. Investigation, code and writing used AI assistance; this is not external peer review or evidence of a fully autonomous Hunter.