After trying a few other things, it seems like you are correct and the spaces became non-breaking, even without appearing as such in the HTML editor or page source. It appears to have been because I copy/pasted "Blockquote " (with space on the end) multiple times to fill as a placeholder while editing in Visual mode. The below test page shows four examples of placeholders:
1) "Blockquote " copy/pasted multiple times while editing in Visual
2) "Blockquote 2 " copy/pasted multiple times while editing in Visual
3) each instance of "Blockquote " handtyped while editing in Visual
4) "Blockquote " copy/pasted multiple times while editing in HTML
2, 3 and 4 worked just fine, but something in 1 ended up thinking it was done with non-breaking spaces. I should be fine going forward, as I'll likely not be making that sort of block of text again.
A strange occurrence, for sure. I will mark this as resolved tomorrow, if there's nothing else thought to be needed for this.