It never existed, at least not in Demigod 1.2 or later. That's why there's no fix for it. Maybe it existed in a much earlier version, but there is no possible/residual code path through which <100% splash multipliers could have negatively affected Tower of Light damage, so I suspect it was never a real issue at all.
The UberFix does make the structure damage upgrades properly apply to Trebuchets as well as forts and towers, however, and it adds a preventative measure that would otherwise cause the tower strength upgrades to reduce trebuchet splash damage. Since trebuchets are completely unaffected by tower attack upgrades in Demigod 1.2/1.3, there's still no negatives to those upgrades, and no reason not to get them.
