Validation fails if ModifyDate and ModDate are specified using different time zones

I have a test PDF with this metadata:

/ModDate (D:20180817115837+02'00')
<xmp:ModifyDate>2018-08-17T09:58:37Z</xmp:ModifyDate>

According to the specification this is valid and should be considered as equivalent:

For properties that map between the PDF date type, defined by PDF
Reference, 3.8.2, and the XMP Date type, defined by Date and Time Formats, value equivalence shall be on a
component-by-component basis, relative to Coordinated Universal Time (UTC), i.e., correcting for local time
zone offset.

EXAMPLE The document information dictionary entries:
/CreationDate (D:20040402)
/ModDate (D:200404080+91132-05'00')

are equivalent to the XMP properties:
<xmp:CreateDate>2004-04-02</xmp:CreateDate>
<xmp:ModifyDate>2004-04-08T14:11:32Z</xmp:ModifyDate>

In org.apache.pdfbox.preflight.metadata.SynchronizedMetaDataValidation#analyzeModifyDateProperty the calendar objects are formatted as strings and then compared:

else if(!DateConverter.toISO8601(xmpModifyDate).equals(DateConverter.toISO8601(modifyDate))) {
  ve.add(this.unsynchronizedMetaDataError("ModificationDate"));
}

In my testcase DateConverter.toISO8601(modifyDate) yields "2018-08-17T11:58:37+02:00" while DateConverter.toISO8601(xmpModifyDate) yields "2018-08-17T09:58:37+00:00". These timestamps are semantically equal (xmpModifyDate.compareTo(modifyDate) yields 0) and therefore the PDF should be considered as valid PDF-A1b.