Just as I was about to send an e-mail message to iHeart Radio's support team earlier today, I came across this post on their help site. I do not know how current any of the information in the post is. According to iHeart, there's a known bug in the Android operating system. The bug was apparently fixed; OEMs have not passed the fix along to consumers. However, disabling "Battery Saver" mode (as is "recommended" under "Android 6.0 and above...") seems to have fixed the issue with all 3 misbehaving apps.
(Note: although "Battery Saver mode" is mentioned in the post, it might be called "Power saving" mode on your phone, as it is on mine.)
Long story short, when you want to use the iHeart Radio, Radio.com or Spotify apps, try disabling "Battery Saver" (or "Power Saver") mode until the issue is resolved. (Just remember to re-enable it when you're done listening.)