Coverage Is Not Fairness: Auditing Learned Controls In Participatory Budgeting

In participatory budgeting, spending-based group fairness scores can report a perfect result even when nearly all voters receive no funded project they approved. We examine one such measure, cumulative share deficit (CSD), alongside voter coverage, and construct a valid allocation with CSD 0.0000 that excludes 99.0% of voters. Next, we hold the weights and feature definitions of learned project scores fixed and replay them through four allocation rules. At the unpenalized score, iterative payment reduces exclusion from 0.1470 to 0.0530 and increases welfare to 1.74x the direct-score value. Adding approval-count completion brings exclusion to 0.0427 and welfare to 1.99x. Neither payment rule produces a statistically detectable CSD improvement. On 18 held-out Warsaw series, learned endowments reduce CSD from 0.1992 to 0.1736, but a post-lock, four-parameter age table fitted on Warsaw training data has lower mean CSD (0.1624). Two locked external tests, one of learned priority and one of endowments on 32 projected-support series in two new cities, do not confirm transfer (decision: falsified). Finally, a post-hoc replay without refitting finds no detectable series-level difference on 58 native-approval series and unchanged allocations on 36. These results support reporting spending-based fairness alongside voter coverage, and learned interventions alongside the allocation changes they produce.