The menu ui on node forms does not validate the menu link content entity

I can also confirm this issue. It affects me as well. Custom menu link content entity constraints are not triggered due to the lack of entity validation.

After some investigation it looks like that the related method is Drupal\menu_link_content\Plugin\Menu\MenuLinkContent::updateLink() which is called by Drupal\Core\Menu\MenuLinkManager::updateDefinition().

I Have supplied a patch that updates the validation in the updateLink and updateDefinition as described above, ofcourse it could be cleaner if it would happen in the form processing, but for now this works. FYI my patch is for Drupal 10.3 so i dont know if it will apply for later version like Drupal 11.