Hi fentum, my understanding of the board game rules is that yes, the Mortar unit gets LOS from a spotting hex. Specifically the rule reads, "The mortar unit has LOS from the spotting hex for all purposes." I interpreted that to mean anything the Spotters can see is "seen" by the Mortar as if the unit was in the hex. Example: for actual firing purposes, i.e. "Direct" versus "Indirect," you use the physical location of the Mortar and not the spotting hex. However for all other purposes, you use the spotting hex. Does that make sense? I attached the full Mortar Rules for discussion purposes.
Now how the PC game interprets this rule might be different. LOS rules are programmed on a map by map bases in a LOS Table the firefight's creator has to setup, so if someone made a mistake, it could vary from one firefight to another (I've found some non-Mortar LOS discrepancies in the past and let Eric know about them). For a final answer to your query, we'll have to wait for Eric's response.