@ejs is right.
Original user data is broken into DTUs (as per @kitz's description page, for which a link was provided by @b*cat), and sent on bearer 0. Any DTUs that require retransmission are sent on the same bearer, with the same amount of FEC and interleaving protection as the original DTU.
Bearer 1 *only* carries management data between the two modems - no user data whatsoever.
Look at those statistics from @ejs again, for bearer 1. The "OR" (overhead rate) is 47.81 kbps down, 31.87 kbps up. Note that the "AgR" (aggregate rate, ie the sum of the overhead rate and the user data rate) is the same value. That is because the user rate is zero.
Bearers 0 and 1 can (and do) have independent levels of FEC, interleaving and retransmission defined. However, I am like most others, and lazily refer to the settings of bearer 0 alone. These are the ones that apply to our user data, and are the ones we care about. And, frankly, account for the biggest volume.
In @ejs' example, bearer 0 runs at 40,368kbps, while bearer 1 runs at just 48kbps - around 0.1% of the total bandwidth. It isn't worth talking about bearer 1. Not unless there is a very specialist topic under discussion.
That last number should offer a further pause for thought. If bearer 1 was used for retransmission, the whole scheme would fall apart if ever more than 0.1% of data needed retransmitting.
