ffmpeg – Picture-in-Picture video time jumps during rotation, when recording with mobile phone

I use my phone for streaming live videos to my server the usual way with javascript and getUserMedia() etc.
Then I’m using FFMPEG to encode it in realtime and additionally put an overlay video onto it.

For some reason, each time I rotate the phone to landscape mode or back to portrait mode, the overlay video will jump forward in time. The overlay video is not beeing modified while the live stream is encoding. It’s just a video file from the harddrive, no stream. Any ideas whats going on?

I tried diferent FFMPEG versions, but the behaviour is always the same.

Orange is the livestream. Green is the overlay.

FFMPEG Command

ffmpeg -re -i strm.webm -i overlay.webm -filter_complex "[0:v]scale=640:480:force_original_aspect_ratio=decreasepad=640:480:-1:-1:color=black[main];[1]scale=iw/1.5:-1setpts=PTS-STARTPTS,format=yuva420p[ovrl]; [main][ovrl]overlay=W-w-10:10[v]; [0:a][1:a]amerge[a]" -r 24 -deadline realtime -crf 22 -cpu-used 15 -level 2.0 -map "[v]" -map "[a]" -c:a aac -c:v libvpx -pix_fmt nv12 -threads 4 -strict -2 -auto-alt-ref 0 -b:v:1 1800k -s:v:1 1280x720 -b:v:0 300k -s:v:0 640x480 -bf 1 -keyint_min 48 -g 48 -sc_threshold 1 -b_strategy 0 -ar:a:1 96000 -scenario livestreaming -look_ahead 0 -seg_duration 2 -remove_at_exit 1 -streaming 1 -window_size 30 -adaptation_sets "id=0,streams=v id=1,streams=a" -utc_timing_url https://time.akamai.com/?iso -live 1 -y -f dash manifest.mpd