What is the cause of rlf due to "PDCP UL release"?

Hello, everyone.
‘PDCP UL release’ is seen in LTE log during data test. What is the cause of the message?

Test Condition

  • ENDC option 3x : Split bearer

Test Scenario

  1. Attach UE
  2. Pump TCP DL
  3. Data transfer well for about 60 seconds. (LTE : 100Mbps / NR : 300Mbps)
  4. Data does not flow after 60 seconds, and RLF occurs after about 10 seconds.