Board Thread:Game Discussions/@comment-63.248.45.107-20170203152140/@comment-27862737-20170203185332

It's not a glitch, actually. DF were fusable prior to the most recent update (late 2016), but are no longer fusable. Furthermore, you can fuse armors with the same elements (both having water, for instance), and the fusion results are vastly different than previous. You can also fuse Starmetal armors now with other armors, and you can fuse regular armors together and get Starmetal out of it.

Furthermore, it was also somewhat "predictable" on what you would receive with your fusion (i.e. fusion a mono water with a mono air would give you mono water, mono air, or water/air), but now it seems to be completely random - I personally fused two Epic armors that both had fire (and no water at all) and ended up with a mono water Epic. Look at the Fusion page for further details.