That is a reply from the Ford Technical Assistance Center (TAC). TAC is there to assist technicians with diagnostics/repairs and to forward engineering issues up the chain. The person responding is not the person who designed the system; they are there to assist the dealer with the repair and only engage engineering when necessary. Since the dealer said the noise "comes from the rear", that would not be IWEs. The dealer also stated that they road tested a similar vehicle and it made the same noise...at that point they need to call it normal, because they have to assume the noise being described is a characteristic. Otherwise, there are dealers who will throw parts at a noise even though it's 100% normal. That is not a well written inquiry to TAC.
The 2019 F-150's were updated with an improved IWE strategy that actually keeps the hubs engaged when cold to prevent partial engagement from low engine vacuum during startup. Depending on temperature, the hubs may not disengage for up to 2 miles. With yours having an issue after being warm, there is a definite vacuum leak.
The TAC inquiry from your dealer shows that they did not do their due diligence. Before contacting TAC, they are supposed to perform Workshop Diagnostics. Does this look like it describes your concern?
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There is an entire 12 Step diagnostic procedure to determine the fault. If your dealer can't do something that basic, I'd find a new one.