If I switch to a different window while in full-screen mode, I immediately lose connection to whatever server I'm on. It's not an issue with my internet, and it doesn't matter what application I switch to. It's frustrating because in the pause menu there's the feedback button that pulls up a page in the default browser, and if you accidentally press it, you're kicked out of the server. This can be reproduced by opening Minecraft Bedrock in full-screen, connecting to a server, switching to a different window, and returning to Minecraft.