I received a CSA 1099R (Statement of Annuity Paid). When I entered the data, TT treated it as though it were a withdrawal from my FERS savings account.

When you added a new 1099-R, did you indicate that it was a CSA 1099-R?

See the three screenshots below.

#1 - where you add a new 1099-R - note that option #2 is checked (CSA 1099-R)

#2 - confirm the payer information - which is already filled in by TurboTax because all CSA 1099-Rs are from the OPM (I hope).

#3 - enter the 1099-R data itself. If you get this far, the program is very aware that you are in CSRS, not FERS.

If this is not what you saw, please delete the 1099-R and go through the process again.