Impossible to say for sure, but you have xhtml (which displays correctly in a browser), ePub and their own in-house viewer. I suspect that at least one of the latter two is not following accepted guidelines.
A half-em space after a full stop at the end of a column of characters is the...
1. I am not a font expert
2. You give no context as to how you are trying to use these fonts (a web page, an app you are developing, something else?)
3. You give no ideas about how you are trying to use them
Having said the above, try the links below and see if they help -...