Gaura lindheimeri.

Happy to share a suggestion. The leaves on yours look more like D. stramonium to me. (And after 2 hours, a wrong suggestion usually has objections.) But for either species, agree, likely not included intentionally, or at all, with your wild seed mix. They just show up sometimes. https://gobotany.newenglandwild.org/species/datura/inoxia/ https://gobotany.newenglandwild.org/species/datura/stramonium/

you can cut the leaves. remove rotting part, let it dry in room temp 3-4 days, then plant it, (S. trifasciata and all Sans that I know of can be propagated by leaves cutting, rearrange leaves in pots as you like. water sparingly, only when medium dried. Room temp bright light but nut direct sun for now. treat like succulent. avoid too much watering. most likely you will end up success.

Cynthia, as Tyler above points out, it's reverting to normal growth. It can easily be grown on its own roots, but was likely grafted to accelerate its growing (which it has done splendidly). Be careful of this plant's sap - depending on your sensitivity, it can be an irritant or worse to skin, and definitely keep it away from all mucous membranes, If you're going to trim it for cuttings (or to take the load off of that graft), have some warmish water to daub the cuts in - that will slow down the milky latex bleeding.