This KBA explains how to subsitute/validate custom fields on cloud systems
SAP S/4 HANA Cloud
Actually the custom fields (which are created in fiori app "Custom Fields and Logic") are supported exclusively in BADI
FIN_ACDOC_EXT_SUBSTITUTION for Creation scenario
FIN_ACDOC_EXT_CHG_SUBSTITUTION for Changing scenario
FIN_ACDOC_EXT_REV_SUBSTITUTION for Reversion scenario
These 3 BADIs are designed to handle custom fields in both IN and OUT parameters.
And in the OUT parameter, these BADIs contain only extension fields.
It needs to create the BADI implementation separately for extension fields.
Custom fields, substitution, validation , KBA , FI-SL-VSR , Validation, Substitution and Rules , How To