It is by design. In the extended tag view multivalue fields are shown with multiple tag values and not with a backslash in 1 field. So if you want to copy and paste a multivalue field in the extended tag view you have to mark these multiple fields and c&p them together.
Always adding a field with c&p would make it impossible to just copy (not add) the content of 1 field to another.
There was a change made with version 3.21a.
I don't know what you mean by "affecting the double-backslash". You still can use a double-backslash in the tag-panel to create a multivalue field.