This known issue is closed due to the decommissioning of Blackboard on the 31st of August, 2021

IE11 will not load custom fonts when navigating directly to pages in Blackboard via the address bar or bookmarks, and will instead display Times New Roman. The issue may also occur when refreshing a page.

Blackboard is aware of the issue and recommends using Chrome, Firefox or Safari to avoid this issue.
Microsoft is aware of the issue and recommends using Microsoft Edge. They do not intend to fix the bug in Internet Explorer 11.

A comparison of the display can be found below, with the incorrect font on the right.