Tiled’s “Isometric” is actually Axonometric and allows more types of projections, of which Isometric is just one case. The exact projection is determined by the aspect ratio of the tiles.
It looks like you entered the wrong tile size, so your tile slots have a width:height ratio of 1:1 instead of 2:1 like your tileset. Go into Map -> Map Properties and set the Tile Height to half of what it currently is. That should fix it!
The tile size in Tiled for isometric maps should be the size of the surface of the tile, ignoring any depth. This will most likely be a different size from the tileset’s tile size. That’s what tends to throw people off, since in the tile sheet, the tiles are often sized differently to give them depth. For most pixel art isometric tilesets, the width:height ratio is 2:1 even if the tile images are square or tall rectangles.