Hi Alex - MapSearch is an online tool used by conveyancers to check to see if a property is registered and if so under which title. It's not intended to be definitive re the registered extent as that's the title plan's role
So on the one hand the buyer's conveyancer should solely be relying on the title plan to confirm the extent and to ask you to check that it matches the reality on the ground/what's being sold
If the seller's conveyancer wants to query the MapSearch extent then they can do so by getting in touch -
Guidance and support for property professionals - GOV.UK
The support team can then check the title plan against MapSearch and amend/respond as appropriate