Numerous questions to the PBS helpline and postings on the FB page would suggest that this is not the case.

Are there times when someone junior got something you wanted? Sure, but in each that I’ve seen it’s been because the senior person didn’t wasn’t legal for the trip in question, had constraints which prevented them from being awarded the trip, or simply didn’t understand what “avoid reserve” actually did. The natural tendency is to then blame the system, of course, but seeing something awarded as PN is a red flag that the user didn’t bid as effectively as they could have. What about coverage dates? Those don’t necessarily honor seniority but again knowing how to use PBS effectively helps mitigates that.

Hawk21 is likely correct in his observation, and I agree with him.