For some reasons (most likely 'pre-mature optimisation') ChildDecorator handles Lists differently from maps and other types of collections: instead of creating an immutable copy of a list filled with proxied elements - an original collection elements are replaced with proxies => the state of the 'singleton' definition re-used by all the users of the webapp is modified.

As of f6ec736175f156c6081e0b58c2b7f8ee763c3462 it causes problems with translating the definitions that have list fields for users with different locales: original def gets partially i18n-proxied forever and gets stuck with a same LocaleProvider.

Video demonstrating the issue: https://nas.lemonize.de:5001/fbsharing/DVjgi2Xn
Patch with an idea how to fix it: https://gist.github.com/p4elkin/cbccff356553fd9b0a87