QB throwing a pass close to the LOS

[North Pole Alaska Ref]

Had this happen last year, umpire threw flag. I (referee) dropped bean bag. Play was away from chains, in side zone, so umpire had to "turn around" to check if it was past LOS. After looking at the bean bag, it was clear the passer was behind LOS. My spot (bean bag) was better. Mechanics book says this is the R's call with assistance from the U/HL. I stress (in Pre-game), the R will make this call, with assistance (only if it is towards chain side and obvious) from U (and then U may throw this flag).