Yes it happens in many countries. For example in UK someone mentioned they have Phenylketonuria and their NHS will not cover a medicine Kuvan that would improves their quality of life by allowing them to consume a reasonable amount of protein.
The NHS Commissioning Board's report on Tetrahydrobiopterin/Sapropterin/Kuvan is here: https://www.england.nhs.uk/wp-content/uploads/2013/04/e12-p-...