If 2 users save their preferences at the same time, the results are unpredictable

The problem comes from https://github.com/xwiki/xwiki-platform/blob/68d16b2ce15aaab838da4d69b4f9cd8f0407316c/xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-preferences/xwiki-platform-notifications-preferences-api/src/main/java/org/xwiki/notifications/preferences/script/NotificationPreferenceScriptService.java#L103

A TargetableNotificationPreferenceBuilder is used to create NotificationPreference objects before the saving operation. However, this component is a singleton, and not thread safe!

A quick fix could be to create an instance of this builder at each request. But I really doubt we need this builder anyway.