You mean it's become an issue that is continually being debated butleft unresolved.
unresolved不是形容词吗，为何会放在left后面呢?
同意曹老师解答看法
You mean it's become an issue that is continually being debated but left unresolved.
备注：that is continually being debated but left unresolved为关系代词that代替前面先行词“an issue” 引导限制性定语从句，关系代词在定语从句作主语成分。
【1】主动结构:主语+leave+宾语+宾补（备注：宾补可以是，形容词，副词，介词短语，目前分词，过去分词）
He left such an important matter unfinished. 他让如此一件要紧的事半途而废了。
【2】被动结构:主语+be+left+主补
Such an important matter was left unfinished by him.如此一件要紧的事被他半途而废了。
【1】主动结构：主语+leave+宾语+宾补
They left the window of the room open.他们让房间的窗户开着。
His illness left him weak.他的疾病让他身体非常虚弱。
【2】被动结构：主语+be+left+主补
The window of the room was left open by them他们让房间的窗户开着。
He was left weak by his illness .他的疾病让他身体非常虚弱。
