In the meantime we tried to implement our own LZ1 uncompressing method – for the time being still no success. Unfortunately there is no any documentation that could clarify some aspects of the Lotus Notes LZ1 method implementation.

Unfortunately I have no info which Notes version you use and the way you export DXL.

The only possible workaround would be to force DxlExporter to always uncompress attachment data. The relevant DxlExporter property takes effect in Notes 8.0 or newer.