It happens with any table that’s wide enough to be horizontally scrollable. When viewing the preview, everything is fine, only when viewing the markdown source in edit mode, it’s not possible to move the scrollbar.
I tested it with this table (it’s the only thing in the file):
| Col 1 | Col 2 | Col 3 |
|------|-------|------|
| This table is very large. Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. | Cell 1 | Cell2 |