Steps to reproduce the issue

Create a plugin - plg_mycomponent_something

XML Manifest looks like this
<extension type="plugin" version="4.0" method="upgrade" group="mycomponent">

PHP code looks like this
class plgMycomponentSomething extends mycomponentPlugin
{
// This will only ever be used by mycomponent when mycomponentPlugin is defined
Some function...
}

Install the plugin.
Changed the version number in the XML.
Use the updater to update it

Expected result

The updater will install the new version

Actual result

Get error
Class mycomponentPlugin not found

System information (as much as possible)

Joomla 4.2.0

Additional comments

Problem cause by lines 587-592 of UpdateModel.php

I suggest adding an Exception catcher like this...

 // If we have a plugin, we can use the plugin trigger "onInstallerBeforePackageDownload"
 // But we should make sure, that our plugin is loaded, so we don't need a second "installer" plugin
 case 'plugin':
  $cname = str_replace('plg_', '', $table->element);
  try
  {
    PluginHelper::importPlugin($table->folder, $cname);
  }
  catch ( Exception $e )
  {
    ; // Ignore
  }
  break;

Those who want to use onInstallerBeforePackageDownload will have to ensure the class mycomponentPlugin gets loaded somehow.