When some students have already been manually allocated to review other students' work, Random allocation fails to incorporate this information properly, resulting in an unbalanced workload amongst students.
Steps to reproduce:
Given a sample Workshop of five students in which all five students have submitted responses:
1. Manually allocate some students. In my case, I allocated: two students to mark one submission, one student to mark two submissions; two submissions with one marker, one submission with two markers.
2. Click Random Allocation.
3. Allocate 3 reviews per submission, being sure not to remove current allocations.
4. Check the allocations on the Manual Allocation page.
Each submission should have three reviewers, and each student should be reviewing three submissions (balanced workload).
Each submission has three reviewers, but one student has four reviews, and another has only two. This is not a balanced workload.
Please note that due to the random element of the allocation alogrithm, this may not be reproducible every time. However, I have reproduced it three times in a row, each time with different manual allocations. I don't think reproducing it should be especially difficult.