Perks Fail to Progress, reset to Zero ( 0 ) or revert to an earlier state

FOR PLAYSTATION Users
If this appears to happen to you on the PlayStation 4 platform this is a visual only glitch and it should fix itself when you next join a server/game. If this does not appear to clear up you should attempt to Rebuild your PS4 database:

https://support.killingfloor2.com/en/playstation-4-knowledgebase/article/rebuild-ps4-database

If your perk level has reverted to an earlier state your save file has corrupted and your PlayStation 4 system has chosen a recent uncorrupted save file to use instead. In some cases if it cannot find a back up, it will start a new save. If this happens and you have a local or PSN backup, you can try deleting the new save and loading the local or PSN backup save onto the system. The game should then attempt to use that save on next load.

If you are a PS Plus member, you can check to see if your data is available for download. You can find that under Application Saved Data Management. You can go to Saved Data in your Online Storage to find it.