Thank you for reporting this bug! The cause might be that the iPhone XR (and other newer iPhone models) only support 48kHz, whereas Music Studio only supports 44.1kHz when used as IAA node or host.
One thing you could try is to update to iOS 12.1.3 which contains audio performance related bug fixes. It’s worth a try, please let us know if this helps.
Round Robin samples are not planned, sorry.
iPhone X/XS/XR screen support is on the list for future updates.