It's not a new change, fields in the Tag Panel are only using the field name without the percent signs. If you change the configuration, it doesn't automatically change what's written to your tags.
You can see in your screenshot of Extended Tag, that the file you've initially tagged with %SERIES-PART% now also has a %SERIES-PART% field. Only after saving with the correct field name SERIES-PART, the change is reflected also elsewhere in the app.
It can be confusing to distinguish between the different configuration options for Tag Panel and File List, so I'm adding a link to the documentation just for reference: