Instagram Video Audio not in synch

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Hello,

I have a specific problem with my uploaded videos. After uploading they are sort of out of synch with the audio. In the beginning its fine but towards the end of the vid they are more and more out of synch. After a few hours this effect seems to be gone so that I think it's something in the instagram processing of the video.
I use h. 265 codec 30fps with the optimal specifications Instagram gives. I've played with other setups but always the same effect. Does somebody have an idea?