1) Open the attached dxf file
2) Right click on "vne_vent_odvod_ec_3f" block in Block list and select Copy block
3) Choose name
4) Insert reference of this Copy block to the modelspace
5) Save the file
6) Close and reopen the file

RESULT: Attributes of block references included in the original block are missing in the copied block.
Immediately after insertion of the copied block I can see in the property editor, that block reference attributes are not "conected" to the block reference, but they are still visible. They disappear when the file is reopen.

Alternative way I use to copy this kind of blocks without error: Open block I want to copy, Select all entities, reference copy of them, paste to modelspace, select them in modelspace and choose create block.