Adjustment clips move around and disappear.

This all depends on where the connection points of the adjustment clips (AC - for short) are.

As far as I can tell, it does not seem to be specific to adjustment clips, but any connected clips should behave the same way (I have not tested this idea thoroughly with other clips, or titles, yet).

Some examples below - clip "P" is in the primary, and clips "A", "B", "C" are connected.

1) Connection point is before the area that will be overwritten:

Pressing Command-Option-DownArrow to overwrite B to the primary, the adjustment clip stays in the same place, as expected:

2) Same disposition, except that the connection point is in the area that will be overwritten by clip B

Before I show you the result, think what you would expect to happen...

Ready?

Do you think that

a) the AC will stay where it is;

b) the AC will move;

c) the AC will disappear

d) the AC will stay but the connection point will change

I have to say that I was expecting c), but instead I got... d)!

So in this case, the effect of the AC does not change.

From several tests, what I get is that the application always tries to keep the position of the AC, adjusting its connection point if necessary, but keeping it connected to the same clip in the primary. Now what happens if the clip in the primary is overwritten completely?

3) In this example, clip "P" is made to completely disappear from the primary, by overwriting A, B and C onto it:

What happens now if we press Command-Option-DownArrow with all three clips selected?

You'd be right if you guessed that the AC will disappear.

Finally, what situations might cause the AC to move?

This happens when the clip in the primary partially remains, but only in areas completely outside the range where the AC is, like in this case:

They are connected to a gap clip in the Primary Storyline. When you overwrite that gap disappears and all connected to it will vanish or move. Hold the tilde ~ key when doing that. Normal behavior when you remove the Primary Storyline clip that has Attached Clips connected to it.

That's strange behavior and not what I would expect. I think you should report this via Feedback if you haven't already. Final Cut Pro->Provide Final Cut Pro Feedback...  or use this link Feedback - Final Cut Pro - Apple

Please show us screenshots of the project and clip frame rate settings (see Inspector). Titles containing adjustment layers may have a different frame rate than the clips. Even if they appear to be the same length, they may not be the same when enlarged. I suspect that the difference in length is the reason the adjustment layer overflows when you overwrite the top clip with the basic storyline.

This is just a guess based on my own experience, so I won't know for sure until I expand the timeline to the maximum and verify it.

You are right. Adjustment clips do behave differently from other clips, or titles. I should have tested this more thoroughly.