ensuring power had been restored， 是带有目的的伴随状语。

现在分词短语，指 ensuring ...这个短语。 ensuring + that 从句，也是个分词短语。如果 ensuring + 名词，也同样是一个分词短语。

并不是因为 ensuring 接了从句，就改变了它的分词性质。

才看到你的追问，已解答。

Building maintenance workers were circulating through the office space ensuring power had been restored, which it had.

1. ensuring power had been restored：现在分词短语。句子主语 Building maintenance workers 是 ensuring... 的逻辑主语。

句子整体意思是：大楼的维修人员在办公区内流动，确保恢复供电。（这些人在流动的同时，确保恢复供电，是带有目的性的伴随状语）

2. 现在分词作伴随状语，也往往带有目的性。例如：

She waited impatiently, hoping he could come earlier.

她不耐烦地等待着，希望他能早点来。

The professor put a finger in his mouth, tasted it and smiled,looking rather pleased.

教授把一个手指放进嘴里，尝了尝，笑了，看起来很高兴。

The secretary worked late into night, preparing a report for the manager.

秘书为了给经理准备一个报告而工作到深夜。

3. ensure 的用法之一是可以接宾语从句，that 可以省略。例如：

The court feels it has a responsibility to ensure that customers are not misled 

法院认为它有责任确保消费者不受误导。

《柯林斯高阶英汉双解学习词典》

We ensure that the work shall be done in the right way. 

我们保证把这项工作做好。

《汉英大词典》