Couldn't really find anything online, but was wondering if anyone else has experienced this. My guild was doing H Blackhand progression and it was my first raid as holy. Whenever we wiped, I'd beacon both tanks, but looking at the logs after the raid, I only had beacon healing on about 4 of the attempts. I'm not sure what was going on as I am absolutely positive I beaconed the tanks before we pulled.
Do you have the log in question? It's easy to check the uptime on beacon from them to see if it is indeed a bug or if it's like Nexii is saying, the tank swapped talent just as you pulled.
I did report a bug where I got 0 healing from Beacon of Light when specced Beacon of Insight, but I guess that's what I get for trying different talents :P
I use a weak aura to alert me when I don't have both beacons out, it helps if you're a dummie like me.
Tanks swapping talents drops your beacon, and is probably what was happening pre-pull. Logs would be great to double-check. Tracking your beacons in your raid frames is a good idea and will help you know who your beacons are on and if they have dropped off for any reason. I do this in grid with a simple icon indicator.
As much as this is annoying, it also happens *sometimes* when tanks get over 60 yards away from you. I've mainly experienced this during the Imperator fight, when we're standing on one side of the room and the tank leaps away to the other end to safely blow up, and it also seems to happen when a beaconed target gets thrown up in the air by Gruul.. or just on trash pulls when the tank goes to fetch the next pack but you're still with the rest of the group. It's insanely annoying, and probably intended behaviour :/