I can't speak to the current version of Laserfiche you are on, but I've tested both cloud and self-hosted  v12 and there is a difference in timing of the two events. I think the incorrect description in the documentation implies you can access the values of the filled lookup fields within this event. In practice this event does not necessarily indicate the form has been filled in its entirety (i.e., 4 columns in a table). I'll figure out if this is intended or not and either fix the documentation or fix the onLookupDone handler. This is actually the intended behavior, I am working on updating this specific documentation page shortly so I'll get the wording/examples fixed as well as provide the expected solution

Regardless, I typically push people to use the onFieldChange function as that not only captures the lookup completion but also any user edited values all in one go. It is slightly more complicated to handle the onFieldChange for multiple fields at once but its realistically only a matter of a few if/then statements.