The way the banner is supposed to work is that it has to be placed well, because otherwise the enemy can capture it and disable it. But for some reason once the SM hits tier 3 and unlocks the healing component of the banner, that mechanic goes out the window since the banner keeps healing regardless.
I've often seen games where the enemy gets baited into a fight to capture the banner and disable its initial shield function, only to lose everything once the heal kicks in despite keeping the banner captured. The strength of that kind of an unstoppable, large AoE heal can make it very hard for the enemy to prevent a push on a shield gen or important territory.
Disable the healing if the chapter banner gets captured. It would give the banner ability the consistency in function it is lacking at the moment, make it easier to understand, and keep the game interesting as the SM would still have to be considerate with its placement even in tier 3.