When a downlink SDU was discarded by the RLC Acknowledged Mode entity due to a full transmit buffer (tx_maxsize), the drop statistics were not being updated.
The `txpdu_dd_pkts` and `txpdu_dd_bytes` counters remained at 0.
It should be noted that while the statistics are named `txpdu_dd_*`, the drop at this stage occurs at the SDU level.