The bug

Each vault can only track up to 128 players, if 128 players have already opened the vault and another new player opens the vault, the first player tracked by the vault will be forgotten and becomes able to open the vault again.

How to reproduce

Unable to reproduce in vanilla singleplayer, unless installing mod such as carpet or play a server with enough players.

Expected results

Each vault can track infinite players.

Observed results

Each vault can only track 128 players.

Code Analysis

Code analysis by Arnav78 can be found in this comment.