This turned out to be an issue with where the initial principal was being set. The application was loading a Login Form from the Shown event of the main form. This Login Form then set the principal upon successfully logging in.
However, I discovered that once the Shown event had completed, it was at this point that the principal was reverting back to the Generic Principal.
Moving everything to the Load event , rather than the Shown, fixed my issue.
I don't know why this was (they are DevEx Forms too if it is specific to this) though!
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