The TSP's Distributions pamphlet (https://www.tsp.gov/publications/tspbk25.pdf) clearly states on page 6:
If you have both traditional and Roth money in your account and are leaving some money in your account, you can specify that your distribution should come only from your traditional money, only from your Roth money, or pro rata.
For tax purposes, how the TSP accomplishes the particular distribution from the various investments in the traditional or Roth account is irrelevant.
