Our guests haven't mentioned the extra money either, but as far as I'm concerned, they are the ones who have had to go through the process of applying for UC, going to the job centre appointments, looking for work etc so that is their money to do with as they please.
I do agree that the point around being able to ask guests for a contribution to costs has not been communicated clearly at all, however, the "thank you" payment is clearly intended to cover costs, as stated on the government page: "We appreciate people’s generosity and do know that there will be costs associated with helping out, and so we are offering an optional ‘thank you’ payment of £350 per month".
I'm not sure why your direct debit has been increasing every month though, I think you need to speak to your supplier about that.
Ours has gone up twice: once when prices went up in October and once due to the increased usage by our guests, which was more than covered by the £350 so really not an issue.