This issue is relatively simple to address as it is actually a setting within word that controls this.
If you follow the below steps, it will fix this problem:
Open workflow maintenance
Locate the word document you were using in the relevant case type
Note down which form is used on the document
Go to the forms tab within workflow maintenance
Locate the form
Open the form in word using the word icon
On the styles pane at the top of your word program right-click the applied style
Select modify
On the window that pops out uncheck Automatically update.
This will stop updates to the letter being applied to the entire style.