New rules registered with the <newRule> keyword don't seem to be activated.
Example:

<configuration>
  <newRule pattern="*/stack" class="...." />

  <stack .../>
</configuration>

This sample configuration produces the following messages when debug is enabled:

INFO ... NewRuleAction - About to add new Joran parsing rule [*/stack, <classname>].
...
WARN ... ImplicitModelHandler - Ignoring unknown property [stack] in [ch.qos.logback.classic.LoggerContext]

After some investigations it happens that the new Action rule is created but not actually registered in the rule store - the code is commented, see SimpleRuleStore#addRule().

As far as I can tell, the SimpleRuleStore now expects rules to be registered using a Supplier<Action> instead of a rule instance. Unfortunately, the <newRule> action is not updated and keeps registering instances instead of a supplier.

This prevents from registering and using custom rules in a Logback XML configuration.