my Live Split is not auto splitting after the fadeout at the end of the game even after the ending song but everywhere else it works fine I cant find any thing on how to fix this.

The run ends when GLaDOS dies, not after the fadeout.

Run should end as soon as the 4th red core is incinerated

should live split stop the timer automatically or do I have to stop the timer?

if you have autosplit activated, you don't need to do any manual inputs with livesplit. it will do everything for you. you can activate this in the edit splits section of your livesplit

ok thanks.