Assuming that nothing is shown in box 5, the code-G Form 1099-R with the box 2a amount being half of the box 1 amount implies that half of the gross amount was directly rolled over to a traditional IRA and half was directly rolled over to a Roth IRA. If the entire gross amount was directly rolled over to a Roth IRA, the Form 1099-R is incorrect, whoever at the TSP told you that it was correct is misinformed, and the Form 1099-R needs to be corrected.
If the TSP refuses to correct the form, you'll need to file a substitute Form 1099-R (Form 4852) showing the correct taxable amount in box 2a and providing explanation.
I recall several posts here about Forms 1099-R being incorrect due to the change in the TSP reporting system.
