Steps to reproduce the issue

JFactory::getApplication()->enqueueMessage('My message', 'My Message Type')

Expected result

Displays
My Message Type
My Message

Actual result

Displays
my message type
My Message

System information (as much as possible)

Joomla 3.9

Additional comments

Suggest changing the function parameters to:

public function enqueueMessage($msg, $type='message', $format='strtolower')

And add a function exists check message enqueue line to:

_if (!empty($type) && function_exists($type))
{
$type = $format($type);
}
$this->messageQueue[] = array('message' => $msg, 'type' => $type);

There are other ways of getting round this, such as in the template, but this change puts the design decision into the hands of the component / module developer.