Similar story happened in school as ta, if a student did something bad or annoyed the TAs, they would Target the student and give a bad or zero grade on problems that can not give a back/white answer. This is usually on marking final exams because few students will review the marking and thus few if any consequences will happen.
Much like UPS targeting boxes, this is 100% not OK, and as a professor, if I found out my TAs were doing this, they would quickly find themselves someplace between "disciplined" and "fired".
I assume you are a responsible professor. From my experience, most professors if not every only cares about their research.
Teaching is like a job not giving them any positive feedback, so why they care?
And TA-ship is usually paid by school meaning no professor can fire TAs.
This is sadly common, but I'd like to assure you there are many of us who are interested in pedagogy, and measure our long-term success on the success and learning of our students.
Are they not anonymous? You might be able to recognise it by the work, but at my university, marking was done by people sufficiently far from teaching individual students to ensure that is not the case.
