The flowers are hard to see clearly, as they are key to plant ID, but this looks like a type of Speedwell (Veronica). About a dozen non-native species occur in Maryland as common weeds, including in lawns. They should not be hard to kill, though it's possible some might be resistant to often-used lawn herbicides by now. Depending on how recently you treated them and what the weather has been like (sometimes weeds need to be in active growth to be affected by an herbicide), it might take several days or a week or more to start dying back.

Veronica usually goes summer-dormant or dies out entirely by summer, so might start yellowing as soon as we return to a stretch of warm weather. If Veronica/Speedwell is not listed on the product label for the herbicide(s) you have been using, consider a different product, or just dig them out if there aren't too many of them. You could always spot-treat stubborn clumps with glyphosate to kill the roots, for example. When Speedwell colonizes a lawn, it might indicate the turf is either under-fertilized or has become too sparse, possibly from lack of overseeding with vigorous cultivars.

Miri