Party buffs aren't hitting all members

Frequency: Daily, multiple times
World name: Balmung
Character name: Aris Frey
Class/level: 50/WHM/SCH
Party or solo: Party, 4/8 people
Example Steps:

1. Cast Protect on stacked party
2. Misses several members
3. Have to cast once or twice more

Summary:
On a regular basis when running 4- and 8-man groups, my buffs (Medica/Medica II/Stoneskin II/Protect) aren’t hitting the whole team. Everyone is within range and not viewing a cutscene. Can happen before and during battle. Sometimes it’s just one player it misses, other times it’s multiple members.

Have noticed it with other healers in various fights, and have spoken to others who have the same issue. I don't believe it's a latency problem because it's happened on several instances before the start of a fight when the party is together and I've asked them to wait, or when we're all stacked.