So...this one is weird because there are two different ways this can be done:

$wgAddGroups['bureaucrat'] = true;
$wgAddGroups['bureaucrat'] = array( 'sysop', 'bot' );

and I'm not sure how we should be merging these...

A task description would be helpful to understand the issue :-)

In T123085#2513354, @MarcoAurelio wrote:

A task description would be helpful to understand the issue :-)

The merge strategy determines how the extension.json system combines multiple array values (e.g. from LocalSettings.php(-equivalent) and extension(s)).

See Manual:Extension.json/Schema#Merge_strategies.