XMPSchemaBasic setCreateDate and setModifyDate don't work if already set

When setting XMPSchemaBasic with in certain PDFs , setCreateDate and setModifyDate do not work if the dates are already set in the pdf.

So this will not work as expected:

XMPSchemaBasic bi = xmp.getBasicSchema();
bi.setCreateDate(someDate);
bi.setModifyDate(anotherDate);

Setting other metadata fields works as expected, except for these two.

I found a workaround . If I set the fields to null, before setting them to a new value everything works as expected:

XMPSchemaBasic bi = xmp.getBasicSchema();
bi.setCreateDate(null);
bi.setCreateDate(someDate);
bi.setModifyDate(null);
bi.setModifyDate(anotherDate);

But still it feels like there is something wrong with the way setting these two dates work.