Comments in PO files can currently be preserved, however they're not actually imported into our archive files, which means they're reliant on the PO files never being lost (or being generated via something else).
We should update the preserve comments setting to actually import the comments into the archives, and then export them again from there when generating the PO files (see ExtractPreservedPOComments).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-82765 in the post.