confused about collective noun:

[hongwang9703]

One In Four People Are Muslim .
i understand that one in four people acts as collective noun and therefore demands Are

but, what about this sentence:

one of the most problematic ethnic groups in the reunified Germany, in cultural and economic assimilation terms, were the former East Germans

wouldnt that sentence require the plural Were as well??

the correct answer is: one of the most problematic ethnic groups in the reunified Germany, in cultural and economic assimilation terms, was the former East Germans