If one of those Mage healers puts Regeneration on a raid member, which puts a 30 second Temporal Beacon buff on the target, then the second Mage cant put his 15 second Temporal Beacon through Mass Regeneration on that target. Basically making that target unhealable for the second Mage unless he also uses the Regneration Rune which not everyone wants. Especially later in the phase when the Raid starts to be easier.
Probably intended mechanic for 2 mages causing a time paradox 