I think this is normal phenomenon.
On board, PSoC5LP works as an UART-USB Bridge, the hardware connection between PSOC5LP and PSOC6 as below figure.
If you don't enable any SCB component in ModusToolBox and assign P5/P5 to it, the drive mode of P5 is Analog High-Z, P5 pad is like break from PSOC6 silicon . As a digital input pin, the drive mode of UART_RX is digital High-Z, which has very high input resistance, very easy to coupling external noise. In this situation, the pcb trace between UART_RX and P5 likes a noise coupling antenna.
If you connect P5 to VDD or GND on board use jumper wire, then this phenomenon disappears.