@mesmoiron
Footnotes’ last update 2.2.5 ? 2.2.8 was to make the URL-wrap applying regex more selective, so it did probably break nothing. (2.2.6 and 2.2.7 were addingand removing an error in that regex; a bug and its reversion.)
To make sure it’s not that regex, please disable URL line wrapping under the General settings tab > Reference container > Allow URLs to line-wrap anywhere: ? No. If nothing changes, the problem was not caused by the last update, and you may re-enable URL line wrapping.
A footnote showing code out of nothing typically occurs when a script has double parentheses. That was the case with a Pinterest share button added among others by a social icons plugin. To check if it’s that, please change temporarily the start and end tag short codes under General settings > Footnote start and end short codes. You may opt for triple parentheses, triple braces, or custom short codes and then type two long random strings in the two text boxes showing up, to de facto disable any usable shortcode, just to see if it makes the fake footnote disappear.
If you would like to share the page of your live website where the bug is happening on your staging site, we’d try to check for any problem.
