Strange occurence with esa reviiew form

I filled in ESA50 review form for my son and subsequently received a letter saying they had looked at the claim and he would continue in the support group. Today 2 weeks after receiving this letter I got a letter saying he had to attend a F2F assessment. I phoned the DWP and they denied ever sending me the first letter and said he had to attend the assessment. Has anybody else had this experience. Thank you