Last month's poll remains in dispute after a political party backed by the junta that has ruled Thailand since 2014 and its main rival both claimed victory.
The youth-oriented Future Forward meanwhile became the third-most popular party in the country, garnering six million votes in the March 24 election thanks to the telegenic appeal of its billionaire leader, Thanathorn Juangroongruangkit.
But legal assaults have trailed his success at the ballot box and could affect final results expected next month, with the Election Commission announcing Tuesday a fresh probe into the party chief for allegedly holding shares in a media company during the campaign.
The punishment could be a disqualification for Thanathorn and members of his party, commission secretary-general Jarungwit Pumma told AFP.
"The decision is still not clear at the moment," he added.
Thanathorn, who is traveling in Europe, said on Twitter that the case was "political sabotage" and he would return soon.
He has a week to respond to the complaint.
His deputy Piyabutr Saengkanokkul told reporters Monday that the shares in V-Luck Media had been legally transferred in January before Thanathorn's run.
"This issue is not a problem whatsoever," he said.
But it could wind up in Thailand's Constitutional Court, which experts say is highly politicised body that last month ordered a ban on an anti-junta party linked to exiled premier Thaksin Shinawatra.
Thanathorn has been hounded by cases and complaints since the election.
Earlier this month he was charged with sedition in a case dating back to 2015.
He is also being investigated under Thailand's Computer Crimes Act for criticising the country's military rulers in a Facebook Live discussion last year.
After the election, Future Forward joined a six-party anti-junta bloc that said it had a majority in the lower house and the right to form a government.