The export is NOT successful. The message says that the AEL code (artwork macro function) for drawing is missing. It seems that this is "coded" layout, not drawn, and you see some cached layout on screen. If the drawing code isn't there, exploding that layout fails, and your layout is empty.
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I don't know
how you broke this, but with the sample LTCC example in my ADS2016 installation, it works perfectly fine: flatten > merge > GDSII and the resulting layout is fine.
View attachment 144544