When record-enabling an audio track in Logic, the fader for that track will change from controlling the playback level into controlling the monitoring level. This makes sense since it is likely that you want to monitor a track at a different level while recording - for example to provide a higher monitoring level to a vocalist during the recording session without the necessity to alter your playback mix levels.

The ProjectMix I/O simply follows the two different modes Logic's on-screen faders can be operating in and moves the level according to what you see on screen. This is feature of the Logic software and expected behavior.