If you have added numbering to the topics with XMind 8 and opened the mind map with XMind 2020, you may fail to get rid of the numbering in XMind 2020 as XMind 2020 does not support automatic numbering feature.
If you need to remove automatic numbering in XMind 2020, you can open the mind mind with XMind 8 and choose None for the numbering in the Topic Format, then save as the file and open it again with XMind 2020.