I can't say for certain if this is the root cause, but I believe I've identified a crucial factor in the current bug with having multiple bards in the same party.
To describe the bug in apt detail, where there are two or more bards in the party, the execution of the skill "The Wanderer's Minuet" by one of these bards will render the skill unusable by any others in the party. Considering the order of songs being played is somewhat important to rotation, this is a rather annoying bug to work around when playing something like a long-lasting alliance raid with another bard in the party.
Was playing bard myself in the new alliance raid, there was another bard in the party who immediately started with Wanderer's Minuet, which obviously meant my Wanderer's Minuet was a tad screwed. So I went into Mage's Ballad instead and just tried my best to play well.
As the fight goes on, I'm annoyingly staring at my Wanderer's button and getting a little upset that I couldn't use it like I wanted. When I noticed it was constantly flashing between the Pitch Perfect button and the Wanderer's button. Meanwhile, the original Pitch Perfect button I had somewhere else on my hotbar wasn't changing at all.
This button was in place prior to patch 6.1 because the button for Pitch Perfect and Wanderer's Minuet were separated. In 6.1, they were collapsed into one button, where Wanderer's would shift into Pitch Perfect while playing the song itself.
When I began my round of Wanderer's, the song button was glitching out as usual, with the other bard playing a different song and our buffs constantly overriding each other. But I noticed my Pitch Perfect button, the original one before it was combined into Wanderer's, was completely fine. It didn't change, and I could use that button to hit my Pitch Perfects reliably.
I'm not a game designer, nor do I know a thing about coding beyond the absolutely bare minimum. But if I'm not mistaken, the songs are functioning normally. Rather, the thing that's breaking is the command for switching Pitch Perfect and Wanderer's properly. Without your own Wanderer's buff to switch into Pitch Perfect, obviously you can't use Pitch Perfect. Hence why other bard songs just screw you out of using it.
Unless you had the original button that was designated to use Pitch Perfect, and not the one that becomes available upon pressing the song.
Because the bard buffs also refresh constantly over the fight, and no two bard buffs are allowed to be applied at a time, it means that the songs constantly auto refresh over each other. And with that, the button for Pitch Perfect and Wanderer's is constantly jumping between which state it's supposed to be in.
Now, I have no fix for this obviously. I don't know the first thing about fixing it. I have a band-aid solution that does require you to play around your inability to activate Wanderer's whenever you want, and it only allows you to use your Pitch Perfects reliably, not function the same as before the patch.
Whatever the issue, my wildest guess is that the command that designates Wanderer's be turned into Pitch Perfect is reliant on what buffs you have applied to you. So any other Wanderer's status besides your own just turns your command into the wrong button. That's entirely conjecture, I could be wrong on that assumption. But from what I've observed, that's my theory.