I have just found the exact same issue on a L series part.

changed

HAL_UARTEx_ReceiveToIdle_DMA(&hlpuart1, RxBuf, RxBuf_Size);

Remove DMA settings entry in .ioc file.

to

HAL_UARTEx_ReceiveToIdle_IT(&hlpuart1, RxBuf, RxBuf_Size);

All now works as expected.