I noticed that on Wayland, Plasma 5, on kde neon 5.21 there are not preview views on the toolbar. As is shown in the picture, it's not shown the correct miniature, in this example i was pointing to Thunderbird, but still shows the Firefox preview. Is there something that i could do to solve this?
I'm using a nvidia GPU, so there the configurations needed to make Wayland work essentially are satisfied (isn't it? i hope not).

It's not NVIDIA specific. It's a known bug, though I'm not sure if it's being worked on.