Attributes panel. Turn off Overprint Fill.
Wow. You're good. Thanks so much. Wonder how that even got selected. Was making me crazy. Thanks again.
You’re welcome. Illustrator won’t let you overprint process white. But if something was a different colour and is then changed to white, any overprint attributes remain. There is no warning except for a subtle icon in the Attributes panel, which, of course, must be visible to help. Years of griping at Adobe have done nothing, because Adobe have no interest in making Illustrator a better program, just in making it a bigger program with more wonderful “features” they can use to trick you into upgrading. Customer service, intuitive software, documentation, support, and even basic product testing are alien to Adobe.
The cause of this is most often black text and logos that are expected to be used for placement in many different circumstances. The designer of the original will add the Overprint attribute to the black objects because the art may be used over other colours. When someone opens the file, then changes the colour to white, they get the problem you had.