I recently ran into this problem with keywords being combined by commas when imported from LRTransporter.
It is odd, because LRTransporter EXPORTS the keywords as comma separated, but expects them to be semicolon separated when importing them.
I do not see an option in LRTransporter to export keywords the way it wants to see them when imported (and this includes dealing with LR's keyword hierarchy).
Maybe I am missing something, but that seems like an odd oversight in a plugin that has been around for so long.
There is a note in LRTransporters plugin management page that says it relies on exiftool for writing out keywords. I haven't looked closely enough at that aspect of exiftool -- maybe it doesn't speak "Lightroom lingo" for keywords.
Work around: In any case, a simple Find/Replace in a text editor of the comma -> semicolon made the import work as expected, with multiple keywords, rather than a single mushed-together bunch of keywords with a comma in them.
I do not rely on keyword hierarchies, so I do not know of a workaround that can handle them.
Just as an aside, I use LRTransporter when I have to move lots of video files from one catalog to another. With jpgs etc, you can write the metadata to the file, and it shows up when you re-import it to the other catalog. But not for video -- LR won't write keywords into video files.