TimeValues that have timestamps such as '+00000001995-00-00T00:00:00Z' with '00' for month and/or day gets converted to '1995-00-00T00:00:00Z' then is passed through MWTimestamp( $timestamp ), which in turn does new DateTime( $timestamp ) for TS_UNIX.
new DateTime( '1995-00-00T00:00:00Z' ) results in an object with date value of '1994-11-30 00:00:00'.
MwIsoTimestampFormatter then considers the year invalid since original '1995' != '1994' and on failure, does fallback to outputting a timestamp.
Example of an item where this occurs on Wikidata is https://www.wikidata.org/wiki/Q4119465
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