BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@Booklet{EasyChair:221,
author = {Tao Xue and Zhaohui Luo and Stergios Chatzikyriakidis},
title = {Propositional Forms of Judgemental Interpretations},
howpublished = {EasyChair Preprint no. 221},
doi = {10.29007/kv25},
year = {EasyChair, 2018}}