The disengagement conditions for RWY TRK will be listed in the Aircraft Systems section of the FCOM. I’m 330, so it might be different to the 320, but my FCOM says:
RWY TRK disengages, when one of the following occurs:
- The flight crew pulls the HDG/TRK knob: HDG/TRACK engages.
- NAV is armed, and the aircraft reaches the capture zone of the active leg: NAV engages.
- The flight crew inserts a DIRECT TO waypoint: NAV engages.
So in your example, it will remain in RWY TRK until you pull hdg etc.