In this case, the target should have rolled Concentration once against the total damage, since the extra damage is not gated behind a save and therefore is part of the Attack's damage.
(Sorry I pasted a screen shot, but it did not get preserved.)
The issue happened on an attack with a Lightbringer Longbow. The extra radiant damage from the Lightbringer is not behind a save, and therefore is part of the damage roll, not a separate source of damage. It should result in a single Concentration check against the total, not separate checks against the Piercing and Radiant damages.
( I can't see the image by the way )
Steam profile : https://steamcommunity.com/id/baraz/
Hey!
The image you posted did not load properly.
Are you talking about Concentration rolls for multiple attacks?