As we added the split callbacks fairly late in the review this got lost a bit.
See
https://github.com/npgsql/npgsql/pull/3678/files#diff-b45ed9d534512c2b95cd6aafbdceff63d87439bf00b57a605d637cc64785e438R498-R502

Here we might be better checking that if one (sync or async) is set the other should as well, this would prevent user pain :p