The help file for "Maths Channels" is either out of date/wrong, or the export operation has a bug!
In the help file"
Opens a .psmaths math channel file and places the math channels it contains in the Library section.
Saves all math channels from the Library section to a new .psmaths file.
But even if you multi select (using the tick boxes, and you can't multi highlight) it only exports the highlighted math channel. I did check the export file, in case it was the import thing going wrong, but only 1 element in the export file.
Equally if you multi select to delete or duplicate, only the highlighted item is actioned, which means the multi select currently has no use ... so maybe more of a bug than help file error?
While it's easy to work around, it would be good if it worked with Multi-select