Eleanor - MapSearch is not definitive re the registered extent/general boundaries and you are already aware of that. The title plan is the definitive detail any seller/buyer should go by.
MapSearch is a tool to enable users to identify registered titles and then obtain and consider the actual registered information, namely the register and title plan.
So they are using it correctly but then going too far by querying/relying on the detail re the actual sale/purchase/conveyancing.
Having said that things should match up so I'd suggest using
our Contact form to flag the issue so the support team can amend the details as appropriate. We can't do it via this forum