change the @online entry type
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For example, you can import from the below and export with BBT to reproduce.
https://scholar.google.com/scholar?q=info:4w4edGgXF20J:scholar.google.com/&oi=gsb&lookup=0&hl=en
I think I found the problem. I was exporting as Better BibLaTex instead of Better BibTex. Using Better BibTex seems to have fixed the issue. But it is still weird that Better BibLaTex would explore 90\% of my articles as @article and the rest as @online.
Unlike BibLaTeX styles, BibTeX styles can't be expected to handle eprint information properly. Therefore, Better BibTeX is exporting arXiv preprints as 'article', with the arXiv information in the 'journal' field. If you prefer 'misc', use this postscript.
See this discussion for more details:
https://forums.zotero.org/discussion/85274/zotero-needs-a-preprint-category