Code B indicates that the plan is not a Roth IRA and is instead a designated Roth account in an employer plan (a 401(k), 403(b), 457(b) or the federal TSP). It's extremely unlikely that a financial institution would make the mistake of using code B for a distribution from a Roth IRA, so it seems likely that you are misidentifying the account as being a Roth IRA when it is actually a designated Roth account in an employer plan.
Distributions from designated Roth accounts in employer plans follow different rules than do those from Roth IRAs. Distributions from designated Roth accounts are always a proportionate mix of basis and taxable earnings.
