I just noticed this yesterday. The way my photo session was evolving, I ended up in manual mode and then went to use live view with the LCD instead of the OVF.
Long story short - the flash did not go off in this configuration after it went off reliably using the OVF and all other factors being equal. I went back to OVF and everything worked as suspected.
Can anyone else please check and confirm this ?
Set up - D850 - manual lens (Zeiss Otus 55mm) Phottix Odin trigger and a Phottix Indra 500.
I also tested with a Nikon SB910 mounted on the D850 camera and same issue. Flash won't fire in live view and manual mode.
A priority also does not fire flash when using live view.
I then checked and with my D810 and I could trigger the flash with live view and manual mode.