Candidate's username being inserted into Onboarding fields, randomly

Summary

When a candidate completes Onboarding forms, their username is being inserted into some fields, randomly

Content

We have had a few - seemingly random - issues whereby the candidate's username is being inserted into Onboarding UDF's, where it doesn't belong.  For example, one candidate's username got inserted into the field for their bank account number.  Another candidate's username got inserted into one of the UDF's related to benefits.

We are struggling to know why this is happening - and then to prevent it.  Does anybody have any ideas?