I'm not entirely sure if this is a bug or not but portable storage interfaces disengage before the contraptions inventory is completely empty.

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

The issue seems to be related to belt speed. It works better with high speed belts but slow belts cause it to disconnect before fully emptying the contraption.

It works perfectly with hoppers though.