I transferred all 1099-B information from my brokerage account electronically with all pertinent information to complete Forms 8949. From what I can see on my completed tax return from TurboTax all Forms 8949 are included, listing all individual transactions. I filed the completed tax return electronically, so I don't understand why the provided Form 8453 has the the box checked saying "...if you elect not to report your transactions electronically on Form 8949." I thought they were electronically filed.
Normally this is triggered by making adjustments to sales summaries, but it sounds like you imported and everything was entered "one by one" at detail level. few things to try:
if you have multiple 1099s, on desktop you can go into Forms mode, on the 1099-B worksheets at the bottom there is a checkbox whether that 1099-B triggered a mail-in to help narrow it down.
on the 8949s check if any items have "see attached statement" in the description.
if you had any non-covered categories this may also be triggering the mail-in tho I have avoided this with input as one-by-one; maybe an oddity of the import, a cost basis is missing instead of 0 etc, tho I'd expect that to get flagged for review