Binary values are garbage collected only if there aren't any nodes referencing them via properties. This appears to be happening in your case after export/import.
There are a lot of fixes/changes since 3.8.1 so the first thing I would recommend is testing your scenario with 4.2.0.Final. If the problem is still there and you can reproduce it in a test case, feel free to log a JIRA and we'll investigate it.
I seem to be encountering the same problem with images uploaded into the DatabaseBinaryStore in ModeShape 5.4.x
If you can provide a test case/sample project that can be used to reproduce this locally, feel free to log a new JIRA. Thanks.