Regarding the TXF format, that is not the issue. I found the following note in the 8949 help doc:
Note: If more than 100 transactions are reported in the Quick Entry Table, transactions for Box A and D of Form 8949 that require no adjustments will automatically be summarized and bypass Form 8949 for lines 1a and 8a of Schedule D. This significantly reduces the size of the tax return which generally makes it easier to print and e-file the return.
