There is not much to this question - I went to edit a post on puzzling to fix a spoiler tag assuming the user messed it up. But to my surprise it was correct. I rewrote the same text as what existed before the edit and it worked fine.
The initial revision of the answer, which had a visible "spoiler" tag, was malformed - as noted in a comment on the answer, as well as noted below on a comment here by Jordan.J.D, there was an extra newline in the middle of the line, splitting it into two lines, which did indeed cause the formatting that was observed. This is not a bug: it's how spoiler tag formatting works, and the behavior seen is correct for what was input. Fixing it by removing the extra newline does indeed spoiler-tag the line as intended.
! I have noted, if it is related, that posting two spoiler tags wihout any seperator character (space and enter do not work) -
! Breaks the spoiler for the whole post.
Until any seperator character is keyed.
! This would work however, if it wasn't broken -
So, there is no way to continue a spoiler for multiple paragraphs unless it is made one block of text.