There is an issue with glfw's input handling: https://github.com/glfw/glfw/issues/747 (https://github.com/glfw/glfw/issues/747)
I experience this on linux too.
Is that fix already included in the latest LWJGL?
No, the latest nightly build is a few commits behind that fix. GLFW and other bindings are going to be updated soon and LWJGL 3.1.2 will be released. But if you really need the fix now, you can use the "Click to browse..." button under the build configurator (https://www.lwjgl.org/download), navigate to lwjgl/nightly/linux/x64/ and download libglfw.so. That file is always synchronized with the latest GLFW commit.
I see, thank you! I love that when I debug hours, then find it as a reported issue :D
I can confirm, that the latest update solved the problem!