Yes, the USART still can receive one more frame data after deasserting RTS pin.
The USART on series 1 devices EFM32PG1 / EFM32JG1 / EFM32PG12 / EFM32GG11 has separated receive / transmit multiple entry buffers, and an additional separate shift registers.
If the receive buffer is full, the RXFULL in USARTn_STATUS and the RXFULL interrupt flag in USARTn_IF will be set to notifying that there is no more space in the receive buffer is available. However, space is still available in the receive shift register for one more frame.
The RTS is an out going signal be controlled by USART peripheral which indicates that RX buffer space is available to receive a frame or not. If the receive buffer is full, USART will deassert the RTS pin, the one more frame data on the line still can be received by the USART since the receive shift register is empty.