A bouquet of flowers to both Brandy and Monica definitely would've made sense.
And whether the picture with Monica was "planned" or not, once it happened, I would hope she'd make it a point to find Brandy and take a photo with her, too. Otherwise it looks like an intentional snub, which matters whether people admit it or not.
Brandy is a veteran in the game, and highly regarded as an artist. Even if Brandy doesn't care, Beyoncé is one of the biggest superstars in the world. Not acknowledging Brandy at her tour but being backstage with others is saying something.