When I access Customizing > Layout> Footer, I made changes to the Copyright field, click Published, it shows the changes on the right hand side’s frontend footer area. However, this changes won’t stay/be saved if I open the site again on other tabs/browsers. This issue only happend only to one site, not on other sites.
I just found out why this happened. The plugin Code Snippets has enabled “footer copyright” function, which overwrited the setting of GP. After I disable it, it works now.