See test introduction_1_02.
DFDL-1707 enabled support for unparsing BigInt's. However, if you modify introduction_1_02 to roundTrip it fails. This appears to be because it is trying to unparse 8.6E-200. It looks like when ICU converts this to text, it results in a zero. Perhaps it's a limit of ICU, or perhaps it's a bug. Should look into if updating to a newer version of ICU fixes this.