Form integrated uploader skip files

[Aphyxia]

Without the form option everything works fine.

With the form option, the items are no longer automatically uploaded and I force them with
onChange: function( filename, extension, uploadBtn, fileSize, file ){ this.submit(); },

This is the problem I face now:
I have 4 pdf test files and a file uploader with multiple selections possible.

1. On first run I select to upload file 1 and 2, in this order, together. Only file 2 gets uploaded.
2. After that I upload file 3 and file 4, in this order, together. Now file 1 and file 4 get uploaded.
   So - file 1 somehow got uploaded later when I selected other files entirely, and this time file 3 was skipped.

My end goal:
User can upload files which are immediately uploaded and are also to be submitted with the form.
To this end form integration is not strictly needed but I would still like to know why this doesn't work.

The backend PHP is pretty much the one from the repository.

[a233894432]

I also encountered this problem, how do you solve it?