OK, done as requested with steps performed below:
1. Deleted catalog.
2. Closed PMP and deleted old log file and checked the directory where the old catalog was and found it empty.
3. Turned on logging
4. Started PMP and created a new catalog in the same location as before. Did not create proxies.
5. Opened Browse and navigated to IPTC | Keywords | <none>
6. Checked two photos had no keywords
7. Added keyword paths to these two photos using the Structured Keyword Panel
8. Clicked <none> to refresh Browse and the two photos disappeared.
9. Closed and restarted PMP and navigated to <none> again under keywords
10. Found two photos with no keywords and added keyword path in the same way as before.
11. Clicked <none> to refresh and the photos remained in the Browse window.
12. Checked that the photos had keywords and they did.
13. Tried to remove these two photos from the catalog and they remained and would not go.
14. Generated log file
Hope this helps