I, too, encountered this issue, but a little quick messing around seems to point the finger squarely at "smart quotes".
To re-create the issue, type some text that uses one or more smart quotes, and then misspell a word after the smart quotes (but not at the end of your text). The red squiggly underline will start two characters after
the beginning of your misspelled word for each smart quote that precedes the word. For example, type:
The "quick" brown fox juuumped over the fennce.
The red underline will start under the 'm' in "juuumped" instead of the 'j', and there will be no underline at all under "fennce" until there are 4 or more characters after it (at which point the underline will begin under the 'c' instead of the 'f'. Any misspellings prior to any smart quotes will be correctly underlined.
Now turn off smart quotes in the editor options and type the same sentence again (including the quotes, which will be regular straight quotes); the underlines will be in the expected locations.
This "off-setting" of the spell-check underlines does add up with additional smart quotes (offset by two characters per smart quote, whether double or single quote), and it is "reset" when you enter a line break to begin a new paragraph. Being a programmer myself, I can't help but offer my hunch that this is most likely a character encoding issue, perhaps due to multibyte character encoding for the smart quotes and the code that's drawing the spellcheck underlines not correctly accounting for them.
For my part, I'll be turning off smart quotes so that I can still see which words I've misspelled but without the underlines "lagging" behind the misspellings. Still a wonderful piece of software, though, I'm glad it's finally making its way to Windows so I can partake in its splendor!