Why do iPhone 16 videos have five audio tracks, four of which are empty?

Spatial Audio is recorded whenever it is enabled in camera settings, you do not need to do anything with the audio mix feature to get it. In fact, the audio mix feature is just for editing the spatial track, it only works if it already exists. See Change sound recording options on iPhone 16 models - Apple Support

When spatial audio is enabled, the iphone records 2 different sets of audio: a normal stereo AAC track as a fallback, and an 1st-order ambisonic track in a new (largely undocumented and seemingly proprietary) format called APAC. The APAC track is the actual spatial audio track. It's actually 6 channels total between both tracks, although if your software understands stereo AAC but not APAC, it might show the two AAC tracks as a single track, making it look like you have 5.

The APAC tracks are not actually empty, just not being parsed correctly.

It is unclear from your post what app you use to edit the videos.