The Copy function has always only copied the fields in the current View for me, including Expression Columns. I have used that quite a bit to get data into Excel I modify Views to get the correct fields and field sequence I want.
I can't really comment on the Paste function, as I haven't used it that often. But I just tested it and i worked fine for [Name], [Album], and [Artist] fields... Ah, why not... I just pasted all available fields from one set of files into another set of totally unrelated files, and that seemed to work fine.
One thing I did notice was that the setting "Allow overwrite of existing values" was unchecked when I first ran the function, so of course, no tags were updated. Once I ticked that checkbox the paste function worked as expected. There aren't any Option setting that affect this function as far as I know.
If the problem wasn't the setting above, there were a couple of changes in MC25 that may have had an effect. The first one may directly relate to the message you are seeing.
25.0.81 (7/22/2019)
13. NEW: Ctrl+Shift+V pasting will accept a series of strings on the clipboard (separated by newlines) and paste them into a single selected field.
25.0.109 (10/2/2019)
2. Fixed: Doing "Paste All From Clipboard" on a list editor in the Tag window would not reliably save the data.