One known reason that produces this problem is when the active SYSLOG has been disabled after an erroneous Z EOD command.
There is NO active SYSLOG to record the WTO.
Once the MVS SYSLOG is re-activated and a syslog hardcopy established the IOATEST ends with RC=0000.
The IOATEST Return Code is in fact the Return Code of the last command entered.
So here the RC=0084 is for the WTO.
The RETURN CODE 84 is in HEXA X'54' and the WTO return code are documented in the IBM manual:
z/OS 3.1 MVS Programming: Authorized Assembler Services Reference, Volume 4 (SET-WTO)
Chapter 35. WTO — Write to operator - page 367
Return and reason codes - page 382
54
Meaning: Environmental error. The message could not be fully processed because of a hardcopy failure.
The message might not appear in the hardcopy log.
Action: Issue a D C,HC to display any active hardcopy mediums. Verify that SYSLOG and OPERLOG are
active and functioning correctly and then retry the request.
The following ONLINE bookmark on IBM doc will point the WTO Return Code:
https://www.ibm.com/docs/en/zos/2.5.0?topic=wwo-description#d87586e2711
NOTE: Internet Links might become obsolete, the manual reference above will help to find the information.