I am using AE CC (18.104.22.1684)
what you should be concerned about more than anything, is that you are using a buggy release. CC 12.0 like every first release, had a few bug updates (and more features you can use) since the first release of CC 12.0 - you should be using 12.2
for getting MP4.
maybe you should try exporting Mp3 through AME and see if you get consistent results. since CC, there was a decision to move the compressed formats from Ae to AME for a reason. the next release lost the ability to compress these outputs altogether using Adobe Media Encoder to create H.264, MPEG-2, and WMV videos from After Effects | Creative Cloud blog by Adobe
How do I set tbn in AE for making mp4 ?
I am not sure these setting are available even in AME. maybe this script can help you: AEmpeg - aescripts + aeplugins - aescripts.com or maybe you can try 3rd party encoder like Handbrake: https://handbrake.fr/ and get better more consistent results
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Thanks for your answer. After upgrade to 12.2, all of tbn value is the same even I don't know why.
Great! helpful information thanks for the feedback. it was probably some kind of a bug fix. you should always use the most recent update of every release to avoid the bugs.