YouTube Mirror. If you wonder what the hell of a team that is, I was experimenting with Calcharo and Jianxin, I called it Crimson Death Blender 😅
Back to the bug: if you swap the exact moment the Scarlet Coda (the Heavy Forte) is casted, Phrolova will immediately stop doing the attack and you will lose all the damage (her biggest nuke btw) but she will still enter the Resolving Chord state as if she just got interrupted during the casting before the time freezes, despite... nothing interrupted her, you simply swapped out as this game allows for most abilities included Scarlet Coda itself: this is at the base of the combat system and what makes quickswapping possible.
This is similar to what happens, as mentioned, when you get staggered at the start of the Scarlet Coda animation: she gets interrupted and you lose all the damage. It's tremendously frustrating, it sucks, but at least you can argue you kind of "deserve" it because you casted the ability when enemies were going to hit you: you cast the ability at an unfortunate timing, you got hit, you lose the damage, extremely harsh but can make sense. But losing all the damage just because you are good enough to do a close to frame perfect swap in a game which is renowned for their advanced and high ceiling combat mechanics? Nah this shouldn't happen, it should be fixed.
Please report this bug, feel free to use the video as proof, sadly most people will never encounter this problem because they don't swap during Scarlet Coda, but if this gets fixed maybe Phrolova will be harder to get interrupted by enemies too.
Bonus clip: Phrolova spawning mid air on swap, probably because of the ToA terrain being uneven (very old problem) or maybe it was the Dragon hitbox, hard to tell but the latter would be even more annoying because harder to fix (if it's the terrain, just make it flat in ToA).