Something dawned on me. Head-slap moment, the exact same issue I had the last time I had to enter in an API key.
When you copy the API key from the initial screen where they "assign" it, and there's a little "copy to clipboard" box you can click, then paste that into the API key field in XF ACP, somewhere along the line it tacks on an extra space. In this case, it actually added a space before AND after the key, in both fields.
Once I removed that extra space, it's working fine.
SMH. Almost 100% sure that originates with google