Subsidence insurance?

I'm aware of someone selling their flat on the ladder, whose buyers have said their mortgage provider won't move ahead until there is insurance in place that covers subsidence. 

The seller has been calling around to ask insurers and most say they won't include this as we are in a high risk area. 

This is a surprise to me, as my nearby flat, which I bought in 2021, was deemed to be in a 'moderate risk area'. My survey said there was no evidence that subsidence was likely to be an issue, and so we moved ahead. And our mortgage provider didn't insist on subsidence coverage being in place. 

Have other people had problems getting insurance for ladder properties that includes subsidence cover? Or a mortgage provider flagging this issue? 

Thanks