Steps to reproduce the issue

• Create a Menu Module
• Add at "advanced" a menuclass (prefix)

Expected result

class name prefix appended
class="nav menu menu-module list-inline d-block myclass"

Actual result

class name prefix appended WITHOUT space, so it will create a new name with the class name in front
class="nav menu menu-module list-inline d-blockmyclass"

System information (as much as possible)

joomla 4.2.8

Additional comments

Fixing takes just a minute...