Additional reproduction notes:
I've had hours/days/weeks of frustration with this bug which has been present the entire time I've used the AVS Audio and Video Editor software.
After spending dedicated time specifically looking into it, I found that this problem, when reproduced as above, actually happens EVERY single time. It is just sometimes masked or not noticeable, or seems random because one of the above steps didn't happen.
To make sure you can reproduce it, here's some parts that I want to put emphasis on, which are stated in the original post, but I want to make sure you understand them.
1)This does not seem to happen when only a single file is mixed. There must be at least 2.
2)In my experiments, this usually does not happen in immediate playback of the mix-Only when the mix is exported/produced. Though, I have seen it happen in immediate playback sometimes. I still don't fully understand why this is. It is possible that this is because other times I have not considered that I might have produced a single file from a mix previously.
3)This bug is not sporadic. It will absolutely happen every time using the steps in the original post. It is not always noticed because the portion of the file which spikes might be very quiet, or be masked by other sounds in the mix, or simply not sound out-of-place. It is most noticeable when it happens in the portion of the file where the dB's are very high, because it goes from silence to very loud. It's less noticeable when it happens in the portion of a file where the dBs are very low because the original volume was not high to begin with, however, in these cases, when you are monitoring with speakers that are particularly sensitive to high frequencies(like ear buds), this spike usually starts with a click/pop/crack, and you can always find it if you look for it.
4)Because this happens at about 0.500 seconds before the file ends, it's cause often gets confused with other processes like transitions(In AVS Video Editor). These additional processes may or may not have an effect on the immediate playback, but in my investigation, they seem only to coincide with and not cause the problem.
Please make this a top priority for fixing because working around it has caused me so much additional work in the last year, making video and sound with AVS software. Your software suites are amazing. They package a lot of high-end features into a very simple and user-friendly interface that I love. A problem like this is so fundamental, that it takes the 5/5 software to 2/5 for such a quality loss in the output.
For other users who are having this problem, here are my work-around recommendations. The sound envelope adjustment feature is the cause of the problem, but it is still so very useful that I wouldn't abandon it. Sometimes these spikes are masked by others sounds or are maybe on-beat(In music) and can kind of blend in. Only look for an alternative when this problem is noticeable.
I've tested the above using the Gain slider feature instead of the red line on the file, and it does not cause this problem. Unfortunately, you cannot shape the gain in a timeline.
It is not so convenient, but do your adjustments manually to individual files in the file editor portion of the software. Rename these custom versions so that you still have your base sound files to work with.