Well aware this is a common issue but would be great to get input from the members here.
My '18 Spider has the common sagging under-halo plastic piece - though it's certainly not as severe as some owner's have shown and posted about, but it's definitely noticeable if you know to look:
It was noticeable enough for my Alfa dealer to take it up with Alfa Romeo corporate and order me a new piece under warranty. Here's where things get interesting...after waiting some months a replacement piece finally arrived from Italy. I made the hour and a half trek to have it fitted...only for the new piece to be exactly the same! The dealership was lost for words - they couldn't fathom how it was designed without any central anchor points to give support.
They took back the new piece and very kindly ordered two more direct from Alfa. The new pieces came in today and comparing them side-by-side they are all identical and sagging in the middle!
At this point I have no clue what to do - besides leaving it as is or asking the dealer if they'll just give me one of the pieces in case mine gets worse. They said they had 2 other 4C's in since mine, and both had the same gap there. Can other members chime in - let me know what you think?