Success! Looks like we've fixed this one. According to our records the fix was applied for
Checkbox boxLabel Wrap 4.2.1
I noticed that in 4.2.1, the checkbox labels now inherited a block display and this caused it to shove the whole label to the next line. This wasn't the same behavior as 4.2.0.
I was able to get the boxLabel to behave like it once did in 4.2.0 by "adding" (actually overriding) the boxLabelCls. This css can be placed in a custom.css file instead of trying to search and replace it in the ext.css.
Thanks for the report. This is a known issue, so I've linked this thread to the bug tracker ID. Looks like it's scheduled to be fixed in 4.2.2.