It's been a problem for some time... What I do is I click on that 'lock' symbol on the top of the Telegram tab. Then I go to cookies and delete them all.
After it's done - I restart the browser and log in to telegram account, authorising it via phone.
As far as I know, no solution has been found yet.
This bug will stay and keep irritating you, so maybe pinned tab with telegram page (web.telegram.org) will be a temporarily better solution.