Hi Will
There was a Note 1692217 relating to the hash tag in the BACS file, also Note 1846064. It does sound like a set-up issue of some kind. For RTI processing, it is really critical to ensure all the relevant SP's / Notes / manual steps for both the payroll side and PI side are completed in your system. If these Notes are already implemented in your system, I suggest check the RTI master note 1808937 in case anything else could have been missed in this area. Also specifically for PI set-up, see Note 1808938.
Best wishes,
Alison