Dear LiamAstle,
Thanks for choosing Microsoft community!
According to your question, if you are using OWA, first please check whether the Use Conversations view in OWA is turned on. To work around this issue, try to turn off the Use Conversations view in OWA and check the result.
Just login the OWA(outlook web app)>click the settings button on the top>select Conversations view>click off>refresh the page and check
You can also refer to this article to get more details: Email message content is missing in OWA (microsoft.com)
In addition, based on your description, I recommend that you switch to another browser and log in to OWA with the same account to check if the same thing happens.
What's more, I want to collect some information so that I can do further research:
Is the same situation in all the emails you receive? Is the same for every email and for messages from different senders?
Please check with the sender of your problematic email to see if everything is fine before it was sent.
If the email body is blank when viewing in webmail, most likely, the email was not delivered correctly to you. It might be corrupted during the transferring process, or the email was incorrectly sent with blank email body by the sender. In this case, you may have to contact the senders to check the sent mail and may also ask them and your email administrators to help troubleshoot this issue.
Hope this can help you, if not, please share me more details.(complete screenshot is appreciated)
Best regards,
Kerry