Two LEDs but they are different in brightness
Your problem is Q2, the buffer transistor on the RS232 Tx/TTL Rx line. Both Q1 and Q2 provide signal inversion, but only the TTL Rx line is being driven by your buffer.

RS232 uses a default high voltage level to indicate a logical zero, so ordinarily (when there is no data being transmitted) TTL Tx and RS232 Tx are physically high. Since TTL Tx is high, there is little drop across the LED and resistor, so the LED is dim. The TTL Rx, though, is being pulled to ground by Q2, so there is full voltage across the LED.