Post your code so we can check your code.
Powering the prototype kit using UART will not work, there is no internal connection. Connecting RX and TX without powering the board may destroy your PSoC. Did you remove the snap-off programming part?
Hi Bob, the snap-off part is still there.
I now put a 10 kohms resistor into the RX line (RX of the PSoC) to prevent energy to come over this path. Now it works as intended. The PSoC does no longer power up when RX is connected.
The problem might be your PSoC getting back powered. The GPIO pins have ESD protection diodes which is causing the backpowering issue. You may use the SIO pins with hot swap option.