You have made posting with no functional area maintained, but YB99 is always generated in postings.
SAP S/4Hana Cloud
In Cloud the substitution is not available as SSCUI.
But behind the scenes in fact there is a substitution active in Cloud: If no functional area was derived from the CO object and also not from the G/L account master data, then this substitution fills the default value YB99 into the functional area field of the journal entry line item
It is standard behavior that YB99 is called, if no functional area is maintained in the master data.
functional area, YB99, substitution, CO object, G/L account master data.
KBA , FI-SL-VSR , Validation, Substitution and Rules , Problem