Flavor, and why it is structural here
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In most categories flavor is added at the end and changes nothing else. In a powder it is several percent of the formula and it alters the physics of everything around it.
Flavor is not a finishing touch
A flavor system in a protein or greens powder can be several percent by weight, which makes it a major component. It arrives as its own particle size distribution, its own density and its own hygroscopicity.
So changing a flavor is changing the blend, not decorating it, and it is one of the changes that should force a retest.
What it does to the blend
It shifts bulk density, which changes what a scoop delivers and how full the tub looks.
It changes flow, particularly where the flavor carrier is fine and cohesive.
And it creates a segregation risk, because a component with a different particle size and density from the bulk is exactly what separates once the tub is handled. A blend that tastes right when mixed and tastes wrong at the bottom of the tub has segregated, and the customer will describe it as inconsistent flavor rather than as a blending failure.
The sweetener problem
High-intensity sweeteners are used at very small percentages, which makes them the hardest component to distribute and the one most likely to segregate.
A tub whose first servings are sweet and last servings are not has a sweetener at the bottom or the top, and the fix is upstream: pre-dilute it into a carrier before adding it to the bulk, and match its particle size to the blend rather than adding it as supplied.
Getting it right
Treat the flavor system as a formulation component with a specification: particle size, density, usage level, and a supplier you have qualified.
Pre-blend the small components. Sample the finished batch from several points and taste as well as assay, because flavor segregation is detectable by mouth long before it is detectable by anything else you have.
And retest when it changes, because customers notice flavor before they notice anything else about a supplement.