The assembly standards commissioner is investigating complaints about MLA Basil McCrea, the BBC understands.
The commissioner, Douglas Bain, is prohibited from confirming or denying the identity of anyone he investigates.
However, informed sources suggested he had received complaints that the NI21 MLA's treatment of staff broke the Stormont code of conduct.
In May, Mr McCrea confirmed he was facing allegations of inappropriate sexual activity. He denied wrongdoing.
The complaints about Lagan Valley assembly member Mr McCrea and a row about NI21's decision to drop its unionist designation underpinned a fierce split in the party which pitted Mr McCrea against his deputy John McCallister.
Mr McCallister has since left NI21 and now sits in the assembly as an independent unionist.
Mr McCrea remains the leader of NI21.
The party also has one councillor, Johnny McCarthy, who secured election on the new Lisburn and Castlereagh Super Council despite the internal party row which dominated headlines in the run up to the May council elections.
The BBC has not yet been able to contact Mr McCrea for his response to the standards commissioner's investigation.