If you attempt to move a node from one site to another, all defined group permissions are lost. This occurs because Alfresco wants to clear Site Groups from the old site, but in doing so, it inadvertently removes all group permissions:

This issue can be resolved by modifying the code to:

if (authority.startsWith(PermissionService.GROUP_PREFIX + "site_") &&

This adjustment ensures that only "site groups" are removed, preserving explicitly defined ones.

Thanks to MorganP at the Alfresco Discord for pointing me to the right direction.