When using a info.magnolia.ui.form.field.definition.LinkFieldDefinition with Multivaluetransformer, the data is saved correctly but an exception is thrown when retrieving it

(see the [ in the uuid
Caused by: java.lang.IllegalArgumentException: [d14950fc-7fc2-4631-92f1-b6f60a1fe40a
	at org.apache.jackrabbit.core.id.NodeId.<init>(NodeId.java:129)
	at org.apache.jackrabbit.core.id.NodeId.valueOf(NodeId.java:52)
	at org.apache.jackrabbit.core.SessionImpl.getNodeByIdentifier(SessionImpl.java:1098)
	... 123 more

OUTCOME
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After some debugging it seems that the issue is not due to the transformer, rather to the lack in our UserManager of a method to save a property as multivalued and User to support such properties.
Regarding the original issue (where the cause was initially thought to be a bug in Multivaluetransformer), a LinkedList object with property values is simply turned into its String representation at PropertyUtil.createValue(Object, ValueFactory) at line #331. This looks like a JCR multivalued property [foo, bar] but it's not (it's a string), hence the failure when retrieving the value.

Whilst it is easy to workaround the limitation of not having a User API allowing to save a multivalued property, an error occurs once one tries to edit/update a User with such a property.

ERROR info.magnolia.cms.security.MgnlUserManager        : Could not retrieve user with name: tina
javax.jcr.ValueFormatException: property /admin/tina/companies is a multi-valued property, so it's values can only be retrieved as an array
	at org.apache.jackrabbit.core.PropertyImpl.internalGetValue(PropertyImpl.java:483)
	at org.apache.jackrabbit.core.PropertyImpl.getValue(PropertyImpl.java:510)
	at org.apache.jackrabbit.core.PropertyImpl.getString(PropertyImpl.java:520)
	at info.magnolia.cms.security.MgnlUserManager.newUserInstance(MgnlUserManager.java:576)

this is eventually due to User (in our case implemented by MgnlUser) not supporting multivalued properties.

PROPOSAL
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Besides having User setProperty(User user, String propertyName, String propertyValue), we'd need UserManager should have a User setProperty(User user, String propertyName, String[] multivalued) as well.
User should have a String[] getMultivaluedProperty(String propertyName) method or similar. Implementations of User should be able to store and return both simple String and String[] (multivalued) props.