try debugging
Class: /SAPSRM/CL_PDO_META_BO_SC_ADV
method: /SAPSRM/IF_PDO_META_CONSUMER~GET_ACTION_METADATA
I do not think it is possible to provide your logic via metadata framework for Add Approver/Above or Below button as the logic is written in standard code.
You may have to raise a OSS ticket to SAP to fix the issue.
Regards,
Sushil.