Basically the title. When writing .. it is converted to …. Also every string being any amount of dots is also being converted to …, even when it makes no sense.
.. -> …
../relative/path/file.txt -> …/relative/path/file.txt
................................... (used as visual separator) -> …
Automatically changing to the otherwise hard to type ellipsis symbol … is a good idea, but everything else should, not be changed.
This happens on the frontend, the characters in the post itself is not converted. Your post looks fine in Jerboa, for example.
That said, I think the dev community should strive to agree on a text display standard that can be implemented across all platforms.