Many crews have their own schemes and plans for the scrimmage kick numbering nightmares and how they are going to be ready for it when the play breaks down. While they may differ in methodology , all I have heard require some crew discussion after the play to verify who was deemed ineligible was truly ineligible. Did not see that discussion here but perhaps they handle it differently.
The absence of a number during the announcement could mean that the fouling player was on the far side of the field and the L was unable to pick it up, or there was so much going on on his own side of the field, he did not mentally note it, or he was not sure but just "knew" someone was downfield illegally. Although Team A ended up kicking the FG on the replay of down, this penalty did erase a 1st down that would have kept the drive for a TD alive. We can argue all day long about how "unfair" that would have been, i.e. Team A screws up the FG attempt but ends up getting a 1st down, but that is the way the prolated spheroid bounces.