It's because the user accepted their own answer.
From the relevant meta FAQ post:
If you accept [...] your own answer: There is no reputation awarded and the answer does not float to the top of the list.
"21 votes remaining" is referring to how many delete votes you have remaining for that day, not the post. That is, you have 21 remaining delete votes that you can use that UTC day.
For more information, see this meta.SE post.
To "pose" a question or a puzzle means to state it, expecting other people to consider the answer.
to assert, state, or put forward
1a : to set forth or offer for attention or consideration
1.1 : Raise (a ...
It might be because of the limitation of the size of our monitor. However as I am not an expert, so I am not sure but when you zoom in the window you get this:
Here clearly, we can see the borders and the same colors.
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 ...