Email CRM In Cases Workflow button can't fill for To email sometimes

I add global rule button on Cases Workflow to send message and send this message on email ,

I add Send email and Fill all boxes in Email But the Box To Email sometimes is empty and sometimes is filling email ,So

I add new field in case for To email and fill this field from my procedure when add case  before click to on send email the field called #case_personemail#

Why sometimes take email on To email and sometimes isn't take email on To email ,Although the CC on email always filling correct 

Please Help me 

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