I've had two girls who both gained weight miserably for the whole time they were purely milk fed. One was bf only, the other supplemented. Both off the chart weight-wise (though very average at birth).
In both cases, we checked out on failure to thrive gain rates, but no action was taken as they seemed otherwise very well, were happy, slept through from a very young age (ie by the age yours is) and generally gave no other cause for concern.
Tbh, 12 oz sounds tons to me, on the basis of my experience, as both had only just reached birth weight at six weeks.
They are now 3.6 years and 21 months, and show no obvious signs of early slow growth. Both eat well and look normal, although neither is tall (but then neither are we).
The only thing that would give me cause for concern would be the amount your dd is able to drink, but I'm sure you've advised your doc/HV of that already. Other than that though, try not to panic and don't let the HV panic you either. We were extremely lucky to have a team who viewed weight gain as one factor amongst several in assessing how well a baby is doing. I bet she'll be fine, and wish you lots of luck.