When adding a SharePoint tab to a channel, it will sometimes fail to load. Sometimes this can correlate with not being logged into the office 365 website through a separate browser. It also does not produce any error, the tab stays blank. Other users have found it works better to add it as a website tab, which prompts for login if the problem occurs. Should the expected behavior be that the tab loads because you are logged into teams and shouldn't need to login again?

This is another thread of users discussing this problem:

https://techcommunity.microsoft.com/t5/Microsoft-Teams/Sharepoint-page-stops-loading-in-Teams-client-Tab/m-p/236076#M16936