ValidHeader IRQ not set in radio

[mrx23dot]

I was wondering why IRQ flag ValidHeader is not set after reception of a valid JOIN-ACCEPT.
Based on debugger we use explicit mode, without CRC in downlink.

DS says:

RX SINGLE mode: 
The receiver will then await the reception of a valid preamble. Once received, the gain of the receive chain is set.
Following the ensuing reception of a valid header, indicated by the ValidHeader interrupt in explicit mode.

Only RxDone is set and checked. Is it a HW bug in SX1276?
Using this flag could prevent extra AES calculations on noise.