Firstly, is the following sentence correct?
My brother is being unusually nervous lately.
If correct, how is being nervous behavior? We usually use the progressive of be to describe a behavior or an action, not states of the mind.
Secondly, I have read in both Longman and the Oxford dictionary that you use only present perfect with lately.
I agree with Possibility. It seems to me that the adjectives that describe some character can denote behavior in this structure (being + adjective), not those which describe states.
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