For players who have entered the Public Test Realm for Patch 1.4.0, an issue has been discovered that causes the Codex of Power to be reset, removing any progress made by players.
After boosting a character to level 100 via the NPC in Kyovashad, the Codex of Power upgrade process will reset to zero. This means that if you have upgraded various Aspects in the Codex and made another character, boosting that character to level 100 will then wipe the Codex to its original state for all characters.
Before boosting a character to level 100 vs after boosting a character to level 100.
The only way to work around this currently if you wish to have numerous characters is to make all of your alt characters and boost them to level 100 before you start to upgrade Aspects in the Codex of Power. This will prevent a new character from accidentally removing all your progress as you battle your way through the Diablo 4 PTR.