During the latest hearing at Insein prison in Yangon on Monday, he was hit with another charge of unlawful association, his lawyer Than Zaw Aung said.
Conviction under the colonial-era law also carries a maximum sentence of three years in jail.
It has previously been used to target journalists contacting Myanmar's myriad ethnic armed groups fighting the state for increased autonomy and control over natural resources.
Fenster's second trial is expected to start on 15 October, Than Zaw Aung said.
His client was "in good health, but he lost weight a little bit", he added.
Fenster, 37, had been working for Frontier for around a year and was heading home to see his family when he was detained on 24 May.
He is believed to have contracted Covid-19 during his detention, family members said during a conference call with American journalists in August.
Myanmar has been in turmoil since the military seized power in a 1 February coup and ousted Aung San Suu Kyi's elected government.
The press has been squeezed as the junta tries to tighten control over the flow of information, throttling internet access and revoking the licences of local media outlets.
More than 100 journalists have been arrested since the putsch, according to Reporting ASEAN, a monitoring group.
It says 48 are still in detention.