Perhaps I do not understand you right, but what you are describing indeed does not sound right to me. Normally, tetra cycles through the available voices. To make sure, try this and see if it works as described:
- use the normal polyphonic mode (make sure you're not in any unison mode), take a patch with long release
- press 1 key: the led of voice 1 should light up
- release key: led of voice 1 goes out, but release is still sounding
- while still in release phase, press a new key: voice 2 led lights up and voice 2 sounds, while release of voice one is still sounding
This is the normal behavior (for example, look at the video where dave smith demonstrates the mopho-tetra polychain: after giving the explanation, he plays a number of subsequent notes, and you see tetra cycling through the voices).
Key assign modes only have functionality in unison mode, they should indeed not impact anything in polymode.