Steps to reproduce the issue

Add a custom modul with a Module Class paramter like XXX under Advanced.

Expected result

the output shall be :
<div class="mod-custom customXXX" ...
or better:
<div class="customXXX" ...
I can't see any benefit to add mod-custom additional

Actual result

<div class="mod-custom custom" ...

System information (as much as possible)

the new code in J4 for mod_custom is in the default.php:

<div class="mod-custom custom" <?php if ($params->get('backgroundimage')) : ?> style="background-image:url(<?php echo $params->get('backgroundimage'); ?>)"<?php endif; ?>>
	<?php echo $module->content; ?>
</div>

before in J3 it was:

<div class="custom<?php echo $moduleclass_sfx; ?>" <?php if ($params->get('backgroundimage')) : ?> style="background-image:url(<?php echo $params->get('backgroundimage'); ?>)"<?php endif; ?> >
	<?php echo $module->content; ?>
</div>

Additional comments

This is my first issue report on github, so I hope this report is okay.