Hi there and welcome to the forum!
Automating the tempo while playing unwarped video clips in Arrangement View is known to cause glitches, since to limitations of Live’s API. The best we can offer for now are a few workarounds:
- Place the clip on an audio track in Session View. Playing unwarped clips in Session View while automating the Tempo should give expected results.
- Use Video Simpler to play back the clip using a MIDI note. Before doing this, make sure to convert the video to HAP, as any other codec is not recommended for use with long duration videos and Simpler. The HAP codec is supported on macOS only.
Let us know if you need any further help with this!