Does this happen if the insert content from your add-on in any other DWG file? Even a blank new file?
Does this happen if they insert content NOT from this add-on?
Is that AutoCAD seat fully updated? ABOUT command is the place to look for exact version to post here if you aren't sure (don't assume, autocad updates do not happen by themselves ever).
Did you reach out to the add-on Support team to ask/make inquiries? Now is a good time to do so.