Maine PTFC senior benefit base never applies (parameter mis-typed as marginal scale)

[DTrim99]

The Maine Property Tax Fairness Credit (PTFC) under-computes the credit for age-65+ filers in tax years 2024+, because the enhanced senior benefit base silently never applies.

Reproduction (single filer, age 65, ME, 2025)

Renter, age 65, taxable_interest_income 17,250.68, social_security_retirement 18,571.52, rent 24,484.64.

Variable	PE (1.749.0)	Correct (2025 Schedule PTFC/STFC)
Benefit base (Sch. line 9)	2,550 (non-senior single)	4,100 (65+, line 8)
Line 11 = base − 4% x income	2,550 − 1,433 = 1,117	3,673 − 1,433 = 2,240
Max credit (line 12)	—	2,000 (65+)
me_property_tax_fairness_credit	1,117.10	2,000.00

Income base (35,822) and rent-equivalent property tax (3,672.70) are already correct; only the benefit base is wrong.

Root cause

me_property_tax_fairness_credit_base_cap chooses the senior benefit base via:

senior_benefit_base = p.benefit_base.senior.calc(greater_age_head_spouse)
senior_benefit_apply = senior_benefit_base != 0
adjusted_benefit_base = where(senior_benefit_apply, senior_benefit_base, benefit_base)

But parameters/gov/states/me/tax/income/credits/fairness/property_tax/benefit_base/senior.yaml is missing metadata.type: single_amount. Its brackets therefore load as a MarginalAmountTaxScale instead of a SingleAmountTaxScale, so senior.calc(65) returns 0 (not 4,100). The guard reads senior_benefit_apply == False and PE falls back to the non-senior single base (2,550).

Confirmed at runtime:

• type(p.benefit_base.senior) -> MarginalAmountTaxScale
• type(p.benefit_base.cap) -> SingleAmountTaxScale (this one has type: single_amount, and cap.calc(65) correctly returns 2,000)

Fix

Add type: single_amount to the top-level metadata of senior.yaml (matching cap.yaml). With that change, me_property_tax_fairness_credit returns 2,000.00, matching the official 2025 Schedule PTFC/STFC. A regression test for a 65+ renter should be added.

Impact

All Maine PTFC computations for filers where the older of head/spouse is 65+ (tax years 2024 onward) — the senior enhanced benefit base has never applied, so elderly filers' credits are under-computed.

References

• 2025 Form 1040ME Schedule PTFC/STFC (lines 7-9, 12): https://www.maine.gov/revenue/sites/maine.gov.revenue/files/inline-files/25_1040me_sch_ptfc_fillable.pdf#page=2
• 36 MRS §5219-KK: https://legislature.maine.gov/statutes/36/title36sec5219-KK.html

Found via PolicyEngine/policyengine-taxsim#1028.

[DTrim99]

Fix up in #8814 (draft): adds type: single_amount to senior.yaml so the age-65+ benefit base loads as a SingleAmountTaxScale, plus an age-exactly-65 regression test. The marginal scale was treating the bracket threshold as exclusive, so senior.calc(65) returned 0 only at the exact threshold — which is why the existing age-66 test passed.

@PavelMakarchuk tagging you for visibility/review.