The problem is in the load() function: MultiVolumeImporter/MultiVolumeImporterPlugin.py at db805948534563566f0a39c63e8d60fac0b63dcd · fedorov/MultiVolumeImporter · GitHub
The attributes come in loadable.multivolume for both sequences and multivolumes, but where the output multivolume is just this input multivolume (with attributes intact), the output sequence is created from scratch and never has the attributes added to it. I’ll fix this and send a PR to the MultiVolumeImporter repo.