I'm not seeing the background issue with your transparent PNG. Your profile says you have Office 2019 - can you confirm the file type is docx and not in Compatibility Mode?

However, since it is a regular shape, you can place your image as a picture fill inside a Triangle shape. This will avoid the need for a transparent image and give you the result you are looking for regardless of the printer driver or whatever else your issue might be.

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Andrew Lockton
Chrysalis Design, Melbourne Australia