Acknowledging the Teaching Manager is being retired, so perhaps an idea for the replacement to not have the same flaw.
As per my own usage and an iSAMS Support article:
if you are moving students to a different set list and the different set is a different subject,
and
if you had selected students before realising the subject was different
choosing a different subject will loose your student selection!
You then need to reselect the students.
iSAMS article referenced - https://support.isams.com/hc/en-us/articles/360018098934-Manage-Subject-Set-Lists
