Tonight, I had a bard cast Hungar of Hadar into a room, then circle back through the dungeon to flank while maintaining concentration on the spell. The spell has a range of 150 feet. Is it possible to maintain concentration on the spell despite losing line of sight and moving beyond the 150 foot casting range?

Concentration was allowed despite exceeding the original casting range. One person questioned the ruling, so I am curious if there is an official stance on the subject.