This is an interesting phenomenon noted for F-35B aircraft during vertical flight and vertical landing. They will do so with their weapons bay doors partially opened.
Of interest, this only occurs during hover (ref 2:48 in the video above) but not in slow flight while the lift fan system is activated.
I have never found the reason why this is so. Does anybody know if Lockheed Martin installed additional auxiliary air intakes inside the weapons bays to provide additional air to the engine during hover?