Is it OK to use loft insulation under roof tiles?

We have been to see a house and when I picked my head into the attic I noticed someone had put loft insulation under the roof tiles (as well as on top of the ceilings). I've never seen this done before. Is it likely to cause any issues? I'm thinking not as it is breathable but, as I say I have never seen this done before.
Thanks.

It depends if the membrane is breathable or not and whether there's a gap between the membrane and the insulation. Our last house had this set-up as the previous owners embarked upon a loft conversion but stopped after fitting the insulation and extending the services up there but it had breathable membrane and a gap, it was all good for the decade we lived there.

Going back in Tuesday for a more thorough check so will get up and check membrane. Have already started the buying process so now looking for issues before it goes to far.