I tested unethical 'experiments' on my kerbals by roping them to some capsules and launching the rockets, but after the stage separation the kerbal stays in place (in the air) and the cable "stretches" for multiple km.

Tested on game version 1.12.3

Could you please attach the logs?

Attached a log and a video of the glitch

https://user-images.githubusercontent.com/76056631/177050635-26574455-dfe6-4ecd-8764-da9a31515f71.mp4
KSP.log