I think I might have figured this out. Originally I had selected to file as a non-resident & part-time resident, which prompted me to apportion income by MA source income while a resident/MA source income as a non-resident. I had interpreted this to mean a split of my income, as a percentage of my time spent living inside and outside of MA. However, I think my mistake was attributing my TX-earned income in that second category (even though my employer is based in MA). Once I reworked my return so that I was filing only as a part-time resident (i.e. I did not earn MA source income while living outside of the state) things seem to be calculated as I would expect.
