MS will not properly investigate breaches of security. What can I do?

The Word behaviour is frustrating, but changing fonts or text size by itself does not prove that another person is controlling the account. It can also be caused by document styles, AutoFormat, add-ins, a shared OneDrive document, or an input-device problem. Treat the account-security question and the Word issue as two separate checks.

First, use a trusted device to change the Microsoft account password to a new, unique password. Review recent sign-in activity, remove unfamiliar devices or sign-in methods, and enable two-step verification. If this is a work or school account, ask the organization's IT administrator to review the Entra sign-in logs and reset active sessions or authentication methods if needed. 

Then test Word with a new blank document saved locally, not in OneDrive or SharePoint. Start Word in Safe Mode by pressing Win + R, entering winword /safe, and typing a few lines. If the problem stops, disable Word add-ins one at a time under File > Options > Add-ins > COM Add-ins > Go. If it occurs only in a shared document, check Share > Manage access and the document's version history. If it also happens in Notepad, test another keyboard or touchpad, because the issue is then outside Word.

Use official Microsoft support for account access and recovery issues, but provide specific evidence: the exact time of an unexpected sign-in, an unfamiliar device, a changed recovery method, or a document-sharing change. If there is confirmed identity theft, financial loss, or unauthorised access to personal data, report it to the relevant local cybercrime or data-protection authority as well.