This started happening after migrating from Qt 5.13.1 to Qt 5.15.0

ColumnLayout
{
    id: layout

    anchors.top: parent.top
    anchors.left: parent.left
    anchors.right: parent.right
}

also happens sometimes on width too.

It seems that in Qt 5.15.0 Layouts inherit anchors properties from Item. Maybe QtCreator cannot find inherited properties for Layouts components.

Also tried fixing by updating from

import QtQuick.Layouts 1.12 to 1.15

This doesn't affect the building process. It only shows up in the editor.