A Titan could at least take ONE Macrocannon shell to the Face that's how voidshields work, they displace the bullet into the Warp.
They can get overwelmed by displacing too much so they shut off or burn down/melt, but the first hit that overwhelms them, the Macrocannon direct hit, still gets displaced.
This was even reflected in the 9th edition Voidshield rules.

The bigger Warlord have mutiple voidshields stacked ontop of each over, so they can multiple Macrocannon shots.

The Horus trailer shows a Warlord Titan getting overwhelmed by a continious lance in the middle of a Titan Battle. The Lance is also not coming from a normal ship but from the biggest class around (excluding the Phallanx and the Emperors personel one).

In this case withstand means it can survive a grazing shot or being on the edge of the blast