Hi xorisa,
Click Edit of the 'Monthly Stats-COD' chart, the chart editor will open in a new browser tab.

Go back to the D365 tab, click New. Then the chart editor will open in a new tab again.

According to the first chart editor, edit the second chart editor to make the Legend Entries (Series) and Horizontal (Category) Axis Labels of the new chart exactly the same as the existing chart.
After saving the new chart, check whether the new chart has the problem you encountered.