If the sign said Sumatra, I think she probably is one. She is just a hatchery-quality one. Hatcheries are rather known for having chickens that aren't quite right for their breed (wrong shape, wrong size, wrong color pattern in the feathers, wrong comb type, wrong leg color, missing crest or beard when the breed is supposed to have one, and so forth.)
As regards face color: Cackle Hatchery has photos of Splash Sumatras that don't have dark faces. Since those are the photos they chose to put on their own website, I don't think they care much about face color.
https://www.cacklehatchery.com/product/splash-sumatra-chicken/
As regards body type, I've never been very good at recognizing that, but I know that many hatchery birds have body types they are not supposed to have. It seems to be a common complaint. So I wouldn't completely rule it out on those grounds either.
And eggs that are actually white is a good point in favor of the bird really being a Sumatra. Most of the Easter Egger types have some amount of brown egg genes, so when they are missing the blue egg gene they lay some shade of brown or cream. Getting actual white is less common.
If the label said Sumatra, I would bet on a poor-quality Sumatra rather than an Easter Egger that lays actual white eggs.
Bresse have single combs, your bird has a pea comb. So she can't be a pure Bresse, and it's unlikely that any store is selling a mix that includes Bresse.
If she isn't really a Sumatra, and you did buy her in a store as a chick, I'm pretty sure Easter Egger (that missed out on the blue egg gene) is more likely than any other kind of mix.
Stores usually buy from major hatcheries, so they have purebreds and they have specific named mixes, but they don't have the kind of mixes that happen if you just throw a bunch of breeds together and hatch the eggs for a few generations. There is always some kind of point to their mixes (lots of brown eggs from most of the sexlinks, colored eggs from the Easter Eggers, a few mixes that grow quickly for meat.)
