CSA Form 1099R Box 2a says unknown instead of an amount like previous years. Why?

There's really no way to tell, unless you contact the Office of Personnel Management (OPM) to ask.  See the office's Get Help page for contact information.

 

OPM doesn't appear to have its manual relating to the completion of Form CSA 1099R available online, but the IRS has its Instructions for Forms 1099-R and 5498 available. The policies would presumably be fairly similar.

 

According to Box 2a. Taxable Amount in the instructions linked above:

Generally, you must enter the taxable amount in box 2a. However, if you are unable to reasonably obtain the data needed to compute the taxable amount, leave this box blank. Except as provided under Box 6 , later, do not enter excludable or tax-deferred amounts reportable in boxes 5, 6, and 8. Enter 0 (zero) in box 2a for:

• A direct rollover (other than an IRR) from a qualified plan, a section 403(b) plan, or a governmental section 457(b) plan to another such plan or to a traditional IRA;
• A direct rollover from a designated Roth account to a Roth IRA;
• An amount from a traditional, SEP, or SIMPLE IRA directly transferred to an accepting employer plan;
• An IRA recharacterization;
• A nontaxable section 1035 exchange of life insurance, annuity, endowment, or long-term care insurance contracts; or
• A nontaxable charge or payment, for the purchase of a qualified long-term care insurance contract, against the cash value of an annuity contract or the cash surrender value of a life insurance contract.

Please see the linked instructions for more information.