@lubos this looks like a bug. i checked this on 18.1.84
the defaults defined for custom fields under Form Defaults for sales invoice do not appear when using Copy To function from a Sales Quote or Sales Order.
the same thing happens when Sales quote is copied to Sales Order and also when Sales invoice is copied to Sales quote or Sales order or Delivery Note. that is all i have checked. i did not check copying to other forms.
the defaults appear fine when creating a form directly.
to summarize, no defaults defined for custom fields appear when using Copy To function.
i am not sure of the other tabs though.