Nested imports (with the children property) are a new feature from 5.0 and certainly not all scenarios have been explored. This could be a source of bugs. In particular, nothing is provided for sorting records if the default ordering (i.e. the order in which the data was read from the external source) is not wanted. Maybe mapping the "sorting" field is enough. I will gladly have feedback from your experience.
In general please report bugs and improvements at: https://github.com/cobwebch/external_import/issues.