While searching on net, I found the answer at https://answers.microsoft.com/en-us/outlook_com/forum/all/email-signatures-disappearing-every-time-i-shut/d2fc96da-0388-4576-9fad-ea7e575153fb?page=2
Microsoft has introduced something called "Outlook roaming signatures" To fix the one needs to create registry described there on the page and it works fine.