I can confirm that it happens when editing the opf file manually. I don't even have to do a file/save. Just clicking on a file in the Book Browser makes the opf change from the inserted:
Code:
<meta name="calibre:series" content="YOUR SERIES"/>
to
Code:
<meta content="YOUR SERIES" name="calibre:series"/>
Inserting the metadata and switching to a different Code View tab and then going back has the same effect.
This being on Windows 10 with Sigil 1.9.20