"issue remains unclear" is correct and usable in written English.
You can use it when you are discussing a difficult problem that has not yet been solved, and remains unresolved or open-ended. For example, "The issue of how to reduce plastic waste remains unclear, as there is no easy solution to the problem."
But a broader issue remains unclear and is being increasingly debated here.
The US famously refused him a visa in 2005, something that still hugely rankled with both him and his supporters, and while the US has since sought to engage with Mr Modi the visa issue remains unclear.
This issue remains unclear and needs to be clarified in the future studies.
Therefore, this issue remains unclear and more data are needed to draw conclusions.
The situation in other chordates, which could help to resolve this issue, remains unclear as we did not find clear orthologs of the vertebrate Egfr ligand in the genomes of Ciona or amphioxus using BLAST and synteny analyses (data not shown).
The mechanistic source for this additive effect remains unclear, but this issue can awkwardly be circumvented by the sequential addition of 1 equiv of 2a after the previous equivalent was fully consumed.
The outcome remains unclear.
The outcome remains unclear, though.
Although a large number of studies have reported the important roles of dendrimer architecture, component, generation and surface functionality in gene delivery, the effect of dendrimer core on this issue still remains unclear.
One crucial issue that remains unclear is whether the two men will acknowledge any misconduct.
Another issue that remains unclear is whether the Justice Department's investigation under Mr. Durham's direction will be limited to the tapes' destruction or will also extend to whether the interrogation methods violated laws against torture.
