Hi Edoardo,
Thank you for posting your concern here in Microsoft Community! I hope you're having a great day despite the trouble you're experiencing with Excel Chart. We tried replicating the issue here and we've experience the same behavior. When we removed the Mar-19 item under Select Data, the Feb-20 data disappeared. See screenshot below:
However, when you include the Mar-19 data, the Feb-20 data will appear on the Data table. You may try it by right-clicking the Data table, and go to Select Data.
Also, we will forward this case to our Internal Support team for further investigation and to get this addressed as soon as possible. We’ll let you know once we reach a resolution. Please standby.
Regards.