The workflow log says user decision is in inbox of user X.
We log in with the user credentials and it is not visible in the inbox.
Then I checked the organizational structure to see if user is assigned to the position and has a validity, this is true. SU53, no dice, also no filters.
Just to be sure I refreshed the HR buffers and also synched and refreshed the other buffers (/$tab and /$sync). Still nothing in the inbox. Executed and debugged SAP_WAPI_CREATE_WORKLIST which does not return the item in the inbox.
Checked table SWP_SUSPEN and transaction SM12, SM58, SM21, ST22 to no avail.
So then I tried to execute the decision in SWIA without check and I could do so. Lastly I forwarded the item in SWIA to another user and voila it appears in the other inbox, and when I forwarded it back again to user X it finally appeared.
It is a rather old system (SAP ECC 6.0 level 11) but still I'm not sure how this happened.
Feel free to chime in on how this anomaly could've occurred.