Hi there. Could you try uninstalling and reinstalling the YouTube app on both the devices (i.e. iPhone & tablet) to see if that makes any difference? Also, try switching network connections and see if that makes any difference. Let us know how it goes.
That's odd. To help the right team better investigate, can you send a feedback report from the device/account having the issue? Steps here: https://yt.be/help/RYwc . You may not receive an individual response, but the details will help us identify and fix any issues, thanks.