[GeoffR]

From what I've seen when using the KePub reader, there are three different issues that affect this:

1. As noted above, the KePub reader treats no-break spaces as stretchable (the ePub reader doesn't).

2. Even without a no-break space after the dash, the KePub reader adds a zero-width stretchable space around certain characters, including em-dash and ellipsis(*). This wouldn't be a problem, except that ... (Edit: It is still a problem, it seems the KePub reader now adds this space around quotation-dash too.)

3. ... The correct dash character to use for quoting text is the quotation-dash (U+2015), but most publishers use the em-dash (U+2014) instead.


(*) There is another KePub bug: the space is only added to the right of the characters, instead of both sides, and this in turn can cause the whole line to be incorrectly justified if it contains one of there characters. But this bug doesn't really affect the OP's problem. This bug can be avoided by enabling the text-rendering:optimizeSpeed; or text-rendering:optimizeLegibility CSS options, but doing so has other side-effects.

(It's the accumulation of all these little typographical bugs in the KePub reader that leads me to prefer ePub over KePub.)