When a component tries to access its own parameters with
$params = JComponentHelper::getParams('com_mycomponent');
within the postflight() method of its own installation script

public function postflight($type, $parent)
{
  $params = JComponentHelper::getParams('com_mycomponent');
}

then the following happens:

• if the system cache is enabled (either Conservative or Progressive) a warning is raised
  Error loading component: com_mycomponent, Component not found.
  

• if the system cache is disabled instead, then the component can access its parameters without problems.

Needless to say, at the time postflight() method is called, the component is installed, and the parameters are available in the database. Replacing the call to JComponentHelper::getParams() with a custom SQL query gets the parameters correctly.

$table = JTable::getInstance('extension');
$table->load(array('element' => 'com_mycomponent', 'client_id' => 1));
// Now $table->params contains the data

This problem affects Joomla 3.7 and newer versions, while the 3.6 series and older worked properly.

I'm pretty sure that this is just another of the problems caused by the well known cache refactoring released with Joomla 3.7.0, and for which a patch is already available at #15875

Technical info:

JComponentHelper::load() does not return the component's parameters
See the line
static::$components = $cache->get($loader, array(), __METHOD__);