The reason is disappearing is because by default the OutsystemsUI on the first child will remove the vertical line

Now to fix this, what I did was override the content: none, but it needs to be tested and see if this is a valid solution or if I am creating more issues. Because of what you are doing I don't guarantee if it will work for your Use case.
But yeah, this seems to work

.wizard-wrapper [data-block*=WizardItem]:first-child .wizard-item-icon-wrapper:before {
content: "";
}
Create a theme on your side and insert this piece of code

To make this work without any repercussions I would put a parent class on the block and override the content: none by using the parent class. This way it will affect only the wizard items that is inside of the block and not all wizard items outside of the block