I am having these issues too.
It is such a pain to stock vendors now I don't even bother.
I need both grid containers and legacy backpacks open at the same time. This is a must for me. I hate how it automatically closes my backpack and containers. When it does that it makes it so hard to find what I needed to stock in the first place because my main backpack is a jumbled mess. This needs to be fixed.
I've also noticed items disappearing as well. I can't see SoTs in my house or on a vendor they vanish. I can mouse over the blank squares and see that they're there but they aren't visible. It's annoying.
I also had my guild sign quest reward vanish on me upon logging in. That freaked me out.
Had that vendor search screen flashing problem before too that was really strange. I couldn't figure it out either.
I've also noticed sometimes I will do a search on the vendor search and nothing will pop up. Then I do the same search again and it pops up. What the...? It used to be reliable and work 100% of the time, now it doesn't.
So now I am so paranoid I gotta run every search twice.
This has nothing to do with the new UI...
They still have not fixed the label problem for legacy backpacks from two and a half months ago. If there are 2 SoTs on a vendor and you mouse over one, then quickly onto another, it will display the same item label, even though one is 0.5 Magery and one is 3.0 Tinkering or whatever. Happens for any items actually. I can be mouse overed a "hammer" then mouse over a "plant" and it tells me the plant is a hammer. It's ridiculous.
It makes shopping and stocking a pain.
I am not trying to be negative or complain or anything. I'm just agreeing yes there are issues and yes they are annoying.