The MODF flag indicates not only the mode fault error but also the underrun error. As in the image you provided the UDRF bit is also set, I think this is the error you get.
This screenshot has been taken from the RX651 user's manual https://www.renesas.com/us/en/document/mah/rx65n-group-rx651-group-users-manualhardware-rev230?r=1054461#G44.6997763
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