If the paragraph where the bibliography is inserted is formated as Bold, which might happen for example if you but the bolded “Reference” heading and then hit carriage return, or if there is no paragraph symbol after the heading “References” which is bolded then the references might come out bolded. If you select the whole of the references including paragraph symbols before and after the formatted references (just to be sure) and unbold everything and then reformat the paper, I think you will find that the appropriate bolding of specific sections will be restored. Alternatively delete the bibliography and make sure there is a paragraph after the title heading is not bolded and is not a “heading” papagraph type (body text or normal).
To see the paragraph symbols, you can click on the reverse P in the word toolbar, which is right in front of the % view box.