That makes sense - you're just producing acid with the bacteria, not alcohol.
You're making a little bit of alcohol too, but usually a lot more acid than alcohol.
IIRC, don't quote me on this, but I believe lactic acid is about 75% the weight of sugar. So if you're expecting it to go from 1.030 to 1.010, that'd be a drop of 20 points. You'd likely only see about 1/4 of that drop, so the FG should be around 1.025. At least, that's the typical attenuation I see when I do that. I've stopped measuring the FG for that reason, and just go by taste.