Thank you for your question, Julie.  Re you able to take a closeup photo of the flower, and one of a stem with leaves?  Does it have below-ground roots (rhizomes) that it uses to keep growing?  Thank you for more information!

Julie:  we are working with a new system.  Can you send me a photo of the flower, closeup?  Is the plant spreading through rhizomes (that look like roots) in the soil?  Thank you!

Thank you.  I still cannot see a flower well enough to count the petals.  Can you please tell me how many each flower has?

Thank you.  I believe this is your culprit:  http://cels.uri.edu/docslink/ceoc/documents/figbuttercup_2010.pdf  I am uncertain why glyphosate (sold as Roundup) did not kill it, bit it may have been applied at the wrong time.  Getting all of the roots and bulblets out is key, and I would think reseeding your lawn will just be a waste of time and money unless and until you can kill every bit of it.  Good luck!