Extend a layer but not its applied behaviour

Ok, I think I see what is going on now.

The behavior is ending at the very same frame as the object.

Notice how only the "Ramp Out" changes, not "Ramp In"?

(by the way, it would change if you were to change the In point, though).

What seems to happen is that when a behavior goes to the very end of an object, it will follow when the end of that object changes.

A workaround: change the behavior first.

For example:

1) move the cursor 1 frame back from the end, select the behavior only, and press O (so now the behavior ends one frame before the object)

2) change the end of the object by pressing O. Notice how the behavior's end does NOT change.

3) adjust that frame from step 1.

Maybe I am not following completely, but this seems pretty straightforward.

Select the layer you want, place the playhead where you want it to end, press O. The behaviors' durations will not be altered:

The rectangle is selected. The playhead is farther to the right. If I now press O, the Rectangle will extend to the playhead position. The behaviors stay:

It seems that the issue was just that you had everything selected, instead of just the object that you wanted to extend. The point is that the O key affects whatever is selected. You can extend everything, or just an object (or several) or even just the behavior(s).

Just to clarify: is it working now?