That's bizarre, as 3dSecure checking comes from the card issuer, so they'd have to be able to get it right otherwise Visa/MasterCard wouldn't sign off the bank using it. AVS CV2 checking is a problem for non-uk cards as some banks only return the security code and don't check the address (Ireland cards are a good example).
With 3d checking Sage Pay sets the checks globally either on or off. High value only with valid 3d checks sounds right, medium value is best allowing 3d and non 3d cards.
For low value payments just don't have a rule. The checks will still take place but you won't filter anything, so no need for a pass, and that way you can still take maesto.
MPI failure is another way to say 'technical problem with the 3d checks'.