What version of filemaker are you using and with what operating system? This appears to be a bug and we'll want that info to make a complete bug report over in the Report an Issue section.
Is the field formatted to be upper case? If so, what happens if you remove the upper case format?
If it is not fomratted as upper case, what happens if you select a different font?
Filemaker 11 Pro Advance 11.0 v3
Using Tohoma font, I changed the style that fixed the problem:
Small Caps - No
Title Case - Yes
Uppercase - No
Lowercase - Yes
Changed font to Arial, results was same as above. Hope this helps.