Hello GiGi,
Thank you for your posting in our forum. To better understand your issue, could you please share more details about “Horizontal Line is not a full line and is inserted centered”? Issue
occurs when composing a new email? Did you mean Insert a line (Outlook->Insert->Shapes)? Here’s my test with 1902 (build 12325.20288) :

If you have different scenario, please do not hesitate to let me know, you can share one screenshot of issue symptom with us for checking. Besides, try to check updates for Office 365 suite
(Outlook->File->Office account->Update Options->Update Now) to see if there’s any new release and symptom would disappear by this way.
Best Regards,
Anna