If just putting conditional visibility on your page element isn't good enough for some reason, I'd recommend placing a container around it and putting the conditional visibility on the container instead. That way all elements inside should be hidden.
“the problem” you describe is unknown to me.
If the input is hidden, but the placeholder is still showing, that sounds like a bug that Mendix should resolve.
Meanwhile. if Eline’s tips don’t help you out, try scss
input:hidden {
display: none;
}
You probably need to add ‘placeholder’ or the dom-object showing it.