I think I might have stumbled on the problem here. Although why I too see your issue yet do not see it using the same code on this site is a conundrum. And then again – maybe I am wrong anyway about finding it. All we can do is ask you to try this. It will involve a small edit to the sp-api-filters.php file.
The function in that file is way down at about line 1940 and is called sp_filter_display_spoiler().
Find the line of code that starts:
$out.= "<a id='spRevealLink'$spoilerID'
You will see that there is an errant and stray apostrophe in there that should not be there. The oine before the $. It should be:
$out.= "<a id='spRevealLink$spoilerID'
Can you make that change and see if that corrects your issue.

