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This is challenging problem.
The time that the K5 switches off is correct, it needs approximately 25 seconds to properly end the video recording. So you have connected the yellow wire to a contact switched connection, this is as it should be.
Can you see in the app which firmware you are running, does this match the firmware you have installed? Maybe something went wrong with installing the new firmware.
I can send you an older firmware version to see if the same problem occurs. This is to rule out whether the problem is due to the firmware. Let me know if you are interested in this.
Before installation I would like to give the tip not to lay the wiring close to the horn. The horn can cause problems due to the strong magnetic field.