You must include the offline_access scope on your OAuth Bot configuration and on bots API permission within your Azure bot registration (Go to AAD - Application Registration (Preview) - API Permission).
Otherwise the bot service will not be able to refresh the token when it expired after 60 minutes by default. https://learn.microsoft.com/en-us/azure/active-directory/develop/active-directory-configurable-token-lifetimes#token-types