This just happened for the first time in PM6. I applied keywords in a few batches to a small set of photos, then closed the panel to resize the preview window. The next time I invoked the Structured Keywords panel (via Opt-Cmd-K, but using the menu has the same issue), it came up with no data, and just the tops of the last hierarchy I applied shown at the tops of the columns. Restarting PM6 fixed the issue.