According to Helix Framework documentation:
Notice! Helix Ultimate template can be installed and used in Joomla 5.1+. But the Joomla core plugin "Behaviour - Backward Compatibility" - must be enabled, with the "Classes Aliases" option enabled, otherwise, Helix Template Options may not work anymore.
And according to the information in the changelog:
Version 2.0.17 (03 November 2023)
Backward compatibility plugin dependency removed
It is not clear to me if the plugin should really be activated or not.
Thank you.