Here are my thoughts. That field you discussed is not exposed natively it appears, which may lead me to believe this was a customization further than just exposing that field on routing lines (5409). There may be code firing as a check to look for lines to validate in that function and this might be a simple validation error at best. Not sure there is enough details here to understand fully if you care to share some more background on what you were trying to solve that is solved but surfaces a new error in PW. Might be able to offer other option(s). If it's development to code, then may need to go back to review that effort with those repro steps that catch that error.
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