I was trying to make a liar puzzle.
This is what it looked like in the preview thing in the edit mode,
Then, this is what it looked like in question.
Is this a problem with my device or the site?
This is a known bug
If you have text right above or below the table, the preview renders the table markdown as a table. But after you save the post, the table is not rendered and just shows the markdown code.
The rest of that answer is a demonstration of the bug.