Universal Credit stopped my son’s Housing costs payments

My son, whom I am Appointee for, is in a flat which his landlord sold to another landlord. When I got the legal paperwork about this I contacted Universal Credit to let them know….this was supposed to be a simple thing as on the change of landlord the tenancy and rent stay the same. But I knew it would be difficult to do a Report a Change form because nothing apart from Landlord’s name was changing. Regardless, in the Journal I was informed I had to do a Report a Change form which I did and uploaded everything they asked for, but it didn’t work. UC said the “Housing costs cannot be paid” and "Housing cost can not be verified as details do not match as on your tenancy agreement. Term dates & Rent amount need to be as on your tenancy agreement.” The tenancy agreement was done when my son moved into his flat 5 years ago so clearly the rent and dates cannot match my son’s current rent. I explained in the Journal that when a landlord sells a tenant’s flat to another landlord the tenancy remains the same - Shelter has information on tenant’s rights when their landlord sells and states "The tenancy continues as normal. No new tenancy or written agreement is required” which I quoted in the UC Journal. I’ve done the Report a Change form now several times and always get “Housing costs cannot be paid” I have phoned the UC helpline and one of their agents arranged for the case manager to phone me but they didn’t. I have asked for help in the Journal about this for several weeks now but no one has helped.

It's my son’s statement day now and UC have not paid any housing costs. I’m speaking to someone in Citizens Advice next week but I cannot understand how UC can stop paying my son’s housing costs - he is a vulnerable adult.