Problem:

1. find also return undefined If it does not find the item.
2. when you search for the undefined index it'll return -1
3. if you pass -1 to splice It'll always remove the last element.

Use array filter,

app.delete("/api/theTasks/:id", (req, res) => {
  const result = Items.filter((p) => p.id === parseInt(req.params.id));

  if (result.length === 0) res.status(404).json({ error: "Item not found" });
  res.json(result);
});

My guess is that the task is not found and index ends up being -1, so you end up calling:

Items.splice(-1, 1);

which removes the last element of Items.

You can fix the issue by checking if the item was found and only deleting it if that is true.

app.delete("/api/theTasks/:id", (req, res) => {
    let taskToRemove = Items.find(p => p.id == parseInt(req.params.id));
    if (taskToRemove) {
        let index = Items.indexOf(taskToRemove);
        Items.splice(index, 1);
        res.json(taskToRemove);
    } else {
        res.status(404).json({ error: 'Item not found' });
    }

});

get index is -1; you should check it before splice; or use filter