Outlook 2019 is only displaying first line of email content

Our client has Microsoft Office Home & Business 2019 installed. Within Outlook, everything was fine until late yesterday. Opening any email in it's own window or viewing in the reading pane, now only shows the first line of the message. Nothing below it (it will list attachments). The message preview option (1, 2, or 3 lines does show the appropriate content) If we access the clients mail on their mobile phone, the whole message can be read with no problems. If we forward an email that only shows the first line from Outlook to another email account, the whole message can be read.

Example - I can send an email to the client listing each letter of the the alphabet on separate lines in the body of the email. Client can open the email in Outlook, but can only see the letter A. Client can view the same email on his phone and see all the letters of the alphabet. If he forwards the email from Outlook to a different email address, the full email showing all letters is visible.

This suggests to me a possible display issue in Outlook but I'm at a loss as to what other actions to take to resolve this issue.

So far I have actioned the following:

1. Updated Office Home & Business 2019 with the latest updates
2. Carried out a local and online repair of Microsoft Home & Business 2019
3. Ran 'scanpst.exe' on local .pst file
4. Disabled the Anti-Virus add-on within Outlook (Client is using ESET)
5. The issue occurs to every email (html/plain text)
6. Ran full AV scan on PC
7. Checked the default font colours

Any assistance would be welcome.

Many thanks