Audacity doesn’t remove those tags, as you will see if you quit Audacity then re-import the exported file.
Have you perhaps updated the version of Windows? 1.2.6 and 2.0.0 both write the “TPE1” frame for “Artist Name” (as it is called in the Audacity Metadata Editor). The ID3 standard http://www.id3.org/id3v2.3.0 defines TPE1 thus:
"Lead performer(s)/Soloist(s). The ‘Lead artist(s)/Lead performer(s)/ Soloist(s)/Performing group’ is used for the main artist(s).
However Windows 7 describes TPE1 as “Contributing Artist”, so if you want to see the “Artist Name” field in the Windows 7 Explorer you will have to display “Contributing Artist” instead.
Windows 7’s so-called “Album Artist” field is the “TPE2” frame which in the ID3 standard is defined as:
“Band/orchestra/accompaniment. Used for additional information about the performers in the recording.”
So I regard that problem as with Windows, not Audacity.
The usual frame for a “BPM” field is the “TBPM” frame (you can open your original MP3 in a hex editor like HxD to see what frame is being used for BPM).
Because Audacity does not have BPM as a pre-defined field it will rewrite the “TBPM” frame as a “TXXX” (user-defined text information) frame. Explorer or Windows Media Player won’t see user-defined fields but any sensible media player or tags analyzer will do so (such as Foobar 2000 or Media Info).
Having said all that, if the “Beats-per-minute” column in Explorer is intended to be the TBPM frame it does not display TBPM data for me on Windows 7 x64.
The “Defaults” button in Audacity Metadata Editor only determines the metadata displayed when there is no metadata in the project already. So if you import an MP3 that has metadata the fields and values set as default won’t appear, as described in the Manual ( Index of /man ).
Gale