ADV7441A STDI_DVALID not available when some RGB timing input

1920x1200 reduce blanking Analog RGB+Sync input,  Sync waveform is normal

User Map, Address 0xB1-0xB5 value as below

read address 0x40 offset 0xb1 len 10!                                                                                                                         

0x0c  0x0d  0x00  0x00  0xcb  0x00  0x00  0x03  0x33  0x20                     

STDI_DVALID not available

So can't identify the input timing

Question, why the input timing is normal, but STDI module not work

Thanks