@Hachmann: Well, I forgot that you are on Linux Mint...
At the release of Ubuntu 16.04 "none" meant actually "XIM". It has been changed via updates, so on an updated Ubuntu 16.04 you have both a "none" option (actually setting "none") and a "XIM" option. I suspect that those changes have not been taken into account on Linux Mint. (They use a modified package.)
@Hachmann: Well, I forgot that you are on Linux Mint...
At the release of Ubuntu 16.04 "none" meant actually "XIM". It has been changed via updates, so on an updated Ubuntu 16.04 you have both a "none" option (actually setting "none") and a "XIM" option. I suspect that those changes have not been taken into account on Linux Mint. (They use a modified package.)
So probably also you had a XIM problem.