Unless Online DRC was switched off, Expedition should have prevented this situation.
Batch DRC should certainly find it in proximity checks.
Please open a service request with your design as attachment, or send me an email with the design.
Layout tool can't check the short because it was designed to check clearance and proximity only. Even layout tool such Xpediton have poweful online DRC and batch DFF check, risks alway exist.
If your plane is static type, Xpedition allow you put any via on the plane whatever the only DRC is on or off. If you run DRC and DFF check, it's possible to report such type of mistake. However, you can't expect layout tool validate all aspects of the design and there are some risks that Layout can't check it.
That's why Valor NPI be used as the golden validation tool.