Acast Open produces a perfectly valid RSS feed in accordance with the technical specifications required to deliver your podcast onto all podcast apps.
When you are submitting your podcast to some services (like TuneIn) they may tell you that your Acast Open RSS feed is not valid. This is because they are using an out-of-date validation tool (like podba.se or w3c) which does not recognize the new Apple Podcasts iOS tags.
If this happens, please tell them:
My RSS feed is perfectly valid, and conforms to the latest Apple Podcasts iOS standards. Please process my RSS feed as-is.