A family of Microsoft word processing software products for creating web, email, and print documents.
It sounds like the Style is set to update base on direct formatting changes made where it is applied in the document content. In the definition of your Style confirm that the box is not checked for Automatically Update. You'll also need to restore the original specifications of the Style.
By way of explanation: If that box is checked any direct changes to a passage where the Style is used will cause updating of the Style & all instances will be updated accordingly.