Regardless of your employer's policy, income you earned from work actually (physically) performed in Wisconsin is Wisconsin-source income, and therefore taxable by Wisconsin.
WI doesn't require non-residents to file a tax return unless their WI-source income was $2,000 or more. But even if your WI income was below that amount, you'd have to file a WI return in order to obtain any refund you may be due. https://www.revenue.wi.gov/Pages/FAQS/pcs-npr.aspx#npr1
ALL your income, including that from WI, is taxable by your home state of NJ. You'll be able to claim a credit on your NJ return for any tax actually paid to WI, so in effect you won't be double-taxed.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
