This occurred in the experimental branch, using a single mod which is attached as well. It adds in new plants which can be harvested to produce procedural metals. It shouldn't have any other gameplay effects. I'm on 51.11 and using DFHack

This save is interesting because a goblin joined my fort in a migrant wave. I went to legends to try and figure out how this happened, but only found more questions, as they appeared to have died in 120, when the current year is 521. They appeared to have died during a raid on a settlement (to a rutherer?) and then did nothing for about 400 years. Interestingly, they appear as female in legends, but in the game they appear as male.