Sorry to hear about the trouble you are facing. I would request you to please share a few more details like:
OS & version of Illustratoror
Is this happening while exporting a specific to PNG, or is it replicable in other documents as well?
Would it be possible for you to upload this document on CC, Dropbox, or Google Drive and share the download link of this file here with us or via DM? If not, please share the screenshot of the object you masked, including this layer on the Layer panel.
We are here to help. We just need more information.