UPDATE 5/12/21: Microsoft should be rolling out a patch soon to fix this.
Microsoft has recently acknowledged they have a fairly widespread problem of the desktop version of outlook failing to display the body of emails properly. If this problem persists for you the work around as listed by Microsoft is to use the website version of outlook. If you don't already have it bookmarked, you can reach this by going to portal.office.com and logging in with your Microsoft credentials.
An alternative workaround is to go to File > Print and the body of the email will be available in the print preview window. Although this is probably more work than the first work around of simply using the web version.
The article from Microsoft where this was publicly acknowledged is here.