Hi! I had a similar problem, and it seems I am not the only one. Yet, this is still a problem in July 2019. I managed to fix it by removing the wps-* files in /usr/share/mime/packages/ and them setting file associations manually. The following post may help you:
https://forum.manjaro.org/t/xlsx-and-docx-files-are-opened-as-zip-file/25814/2It is tailored for Manjaro users, but the same is true for Ubuntu and Mint (at least).
Hopefully, the good people in charge of WPS Office will fix this issue.