I hid subsites and pages from global navigation and manually added links in global navigation. I have created pages in pages library and added links to these pages in global navigation. Strangely link to one page is automatically removed by sharepoint online everytime I add it to Global navigation, this does not happen for links to other pages. I am not able to find cause for this issue.
Link that is being removed for specific page is : https://server/sites/sitecollection/Pages/Clerk.aspx
Dhaval Raval