Altium Designer: How can you extract single members of a Harness?
You can't do it like this. If you have a harness connector it MUST always be completely the same in your schematic.

Yet, you could easily add a non-connected harness wire and name it COM_IF_NAME.I2C supposedly you give the harness wire the name COM_IF_NAME. Make it a habit to also give harness wires a net label and such a defined name. 

Be aware that the harness type is different from the name. You can have multiple times the same harness connector type but connections will be made through the name.

E.g. you have a harness type UART which has 4 signals (RX, TX, RTS, CTS). You can name the harness wire UART1, UART2, UART3, ... and they will be seperate nets albeit sharing the same type.

In your schematic, you named your UART signals in a strange way. There is no need to add indices to them as the internal name will already be COM_IF_NAME.UART3.UART3_TX. Do yourself a favor and just name the harness entries TX and RX so the resulting name is COM_IF_NAME.UART3.TX.