It does that with mine as well stickers, though sometimes past messages are still there - and I'm still on the old layout. PITA

Most buyer feedback I leave via selling manager in bulk, and you can store comments in that (it's also still keeping previous comments, but AFAIK that's a browser function rather than something eBay does, as you can select to clear all history when you clear cookies, cache etc - obviously there's something about the new form that doesn't allow this, though). 

Seller feedback, I usualy write one comment then copy/paste, if appropriate. 

I'm also not getting that dropdown on previous feedback comments, which is annoying. If I buy multiple items from a seller, I generally leave a different comment for each item. Often the same thing, but worded differently. I still like to reuse those comments sometimes for other sellers. Same with buyers. If they've bought multiples, I like to leave a different message for each item. I like to reuse some of those messages later.

I think it's to enhance our eBay experience.