Dear Devs,
I believe I found a bug that results in permanently missing an level in your elements:
By "upgrading" your worker via ctrl+skill-hotkey after receiving 1 lumber you lose the level-up button. Lumber is still there but you can't upgrade your element. On your worker it shows the element level upgraded but at your well on the top it does not. It is also not available for building your towers.
Only works on an element that you have at least 1 level in already.
Cheers,
1401Ger