The label text has been entered in the datasheet, not in the text label in the chart itself.
Therefore, it acts as if it were a single character.
This is called a text field.
Any text or number in the chart that has been generated from data in the datasheet exhibits this behavior.
You can edit the label text in the datasheet. Double-click the chart to open
the datasheet. Use Alt + Enter
to insert a line break in Excel.
Alternatively, you can enter label text directly into the chart labels
on the slide. This gives you more control over line breaks and formatting.