Research on pedagogical benefit of lab reports at the graduate level?
There is research to graduate student's teaching skill:


Graduate Students’ Teaching Experiences Improve Their Methodological
Research Skills (link)


And research to lab reports, evaluating the reports and how reports are viewed:


Using the Science Writing Heuristic in the General Chemistry Laboratory To Improve Students' Academic Performance (link)
Development of a ‘universal’ rubric for assessing undergraduates’ scientific reasoning skills using scientific writing (link)
“As You're Writing, You Have these Epiphanies” - What College Students Say about Writing and Learning in their Majors (link)
The Laboratory Report: A Pedagogical Tool in College Science Courses (link)
Writing-To-Teach: A New Pedagogical Approach To Elicit Explanative Writing from Undergraduate Chemistry Students (link)


If a paper you found is interesting, use Google Scholar to search for that title and click 'Cited by' to see other papers that refer that paper. Then you might find more interesting papers. For example, for papers that cite item 3 probably use this rubric or relate to the use of lab reports: (link).

I'm not in this area of research, so it's hard for me to find the right papers and I'm not 100% sure what you look for. Some of the links above require university access to read the full paper.