Will leaving Thailand before my first exit stamp on a METV create issues upon re-entry?

You have a 6-month, multi entry tourist visa, which lets you enter Thailand all you want during the validity of the Visa and each entry will get you 60 days.

Doesn't make a difference when your exit stamp is dated because the second you leave Thailand that stamp cancels, and when you come in you present the PDF print out of the METV along with your passport and they will stamp you in for another 60 days no problem.