OK, I got it. Token 1190388.
In this test file, the MAGI shown on the Roth IRA Contribution Limit Worksheet should be $250,000, not $50,000.
Recreating the problem required including a Form 1099-R for a Roth conversion from a traditional IRA and having basis in nondeductible traditional IRA contributions. Got lucky in being able to initially see this symptom since my own personal tax return includes the necessary elements.
