Uppercase words with non-alphanumeric symbols, e.g. period,  are recognized as spelling errors by the spell checker.

There are two scenarios which we think are related to this.
1. For example, you have  U.N. spell checker is recognized only the U.N and considering the last dot as an end of a sentence. If you add the U.N. to the dictionary the spell checker still recognizes only U.N and the last dot is ignored.
2. The SpellCheckUppercaseWords property of the DocumentSpellChecker is set to false and words with uppercase should be ignored. Nevertheless, this property is set to false the U.N. is not ignored.