Loosing genlock when using loadbg

Hi guys,

I use a decklink 4k extreme to produce graphics in 1080i5000 and it works well.
However, as i send 'LOADBG 1-5 DECKLINK DEVICE 1 FORMAT 1080i5000', i loose my genlock signal.
I know that because the Blackmagic desin application 'Desktop Video Utility' shows that the card is locked on ref input before sending the command and not anymore after... and as i send 'CLEAR 1-5', the card is locked again.
It looks like showing the video input prevents the card to be locked on genlock signal.

Did anyone knows why ?