Ubuntu thinkpad e15 gen2 lid and fn keys not working
After suspend, all the fn buttons start working. It's not a solution but can help a little bit.
A bit late maybe, but Lenovo confirmed this bug a few weeks ago and has released a new BIOS update yesterday that fixes this issue. I've installed it and it works fine now. You can download the update on the official Lenovo website, it should work smoothly after that.
Note that this update is primarily rolled out for Windows, so you have to install the update via the provided ISO file. You can do this with a bootable USB, see here for an example.