I asked about this awhile ago u
Interrupts with UART wit Arduino R4 - UNO R4 / UNO R4 Minima - Arduino Forum
And @ptillisch replied:
As for the other stuff with the Serial objects. There are lots of issues with them, which I have posted about. I have tried to rework the serial implementation, and have a now probably very stale PR.
Serial: Tx use FIFO plus fixes by KurtE · Pull Request #90 · arduino/ArduinoCore-renesas (github.com)
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