The income should be apportioned as if you were still commuting to Massachusetts.
From Massachusetts Form 1 Instructions, page 3:
2020 Revised Guidance on the Massachusetts Tax Implications of an Employee Working Remotely due to the COVID-19 Pandemic
In response to the COVID-19 pandemic, several states, including Massachusetts, have declared a state of emergency, and many businesses have implemented work-from-home requirements for their employees in response to government orders and public health recommendations.
Due to Massachusetts’ COVID-19 state of emergency for tax year 2020, all compensation paid to non-residents, who would generally perform such services in Massachusetts but for a Pandemic-Related Circumstance, will continue to be treated as Massachusetts source income subject to Massachusetts personal income tax.
Additionally, in consideration of the fact that other states have adopted similar sourcing rules, for tax year 2020 Massachusetts residents, who, immediately prior to the Massachusetts COVID19 state of emergency were employees engaged in performing services from a location outside of Massachusetts, and who began performing such services in Massachusetts due to their employing state’s COVID-19 state of emergency or other Pandemic-Related Circumstance, will be eligible to claim a credit for taxes paid to that other state, to the extent generally provided under Massachusetts law. For more information about this change go to mass.gov/dor.
