Normalization should happen on update as well as on save.

[LiFaytheGoblin]

for that to happen one needs to pass { runValidators: true } when calling an update method.
However, validation can also be forced using hooks

https://delventhalz.github.io/javascript/mongoose-hooks/

[LiFaytheGoblin]

would need to happen on each update hook:
update, updateOne, findOneAndUpdate, findByIdAndUpdate

Which fields does this concern? Not many?

Company.emailAddress,
Internship.startDate,
Internship.endDate,
PdfDocument.path
User.emailAddress

Unsure if not covered already:
Company.mainLanguage, Company.Size, Internship.paymentSize

Not, since only save possible:

StudentProfile.studentId, Supervisor.emailAddress

[LiFaytheGoblin]

check again before release