I have a catalog that has the IPTC keywords 'American Kestrel' and 'American kestrel'. Searching on the terms in the Search tab without regard to case returns the correct number of images - 305. Using case-sensitive returns 2 for the first keyword string and 303 for the second keyword string which is correct.
The Filter tab by IPTC/Keywords ONLY shows the 'American Kestrel' spelling (2) and none of the 'American kestrel' spelling.
Changing the spelling on one of the image keywords properly displays in the Search tab but the IPTC / Keyword on the Filter tab does not update.
Running a reindex to see if that resolves the issue.