ADV7393 as black burst generator but problem locking to external H/V

I've seen the message: Using ADV7393 as black burst generator

I want to do the same, I'm using slave SD Timing Mode 1 (using HSYNC and Field), connected a 27Mhz clk and left the pixel data P[7:0] connected to GND.

But i have a problem getting the analog video output to frame align on the sync inputs. It is locked to the pulses but it starts random not locked to the Field. I've tried reseting the internal SD timing from the ADV7393 by setting bit7 from register 0x8A.

But then it randomly starts with its frame

Best Regards

Teun