Picard provides two tags: originaldate and originalyear.
originalyear is just the year part of originaldate. It depends on that tagging format what gets written where. For ID3 it stores originaldate to TDOR (since this supports YYYY-MM) format.
Actually IMHO it should not store originalyear to ID3 at all. Originally Picard didn’t do this, but at some point the default behavior changed to write all otherwise unknown tags to TXXX: frames. This is a side effect of this.
If it bothers you under the originalyear variable:
$unset(originalyear)