When word contains non-letters symbols at its start and/or end, it's always considered incorrect by the spell checker, even when it's present in the spell checking dictionary. The most common case is with abbreviations like "dr.", which should be invalid when are typed without the period ("dr"), but valid otherwise ("dr.")

Partial workaround: add the word without the punctuation to the dictionary. The problem is that the word without the punctuation will also be considered correct.