Received WCA telephone appointment but haven't filled out UC50

My partner was the lead claimant on our couples claim for ESA. We were migrated to UC in February and my previous status as support group didn't carry over to lcw/ra. It never got resolved and citizens advice and welfare rights didn't believe it was worth pursuing to tribunal. So I've been stuck handing in fit notes and doing work coach video chats once a month. No other requirements.

At last work coach meeting last week she asked if I'd received UC50 yet and I said no. She checked and saw none had been posted to me. After the meeting she messaged me on the journal and said she posted one and I had 4 weeks. I received it the other day. Today I got a letter in the post saying next week I had a telephone appointment with maximus for a WCA. My partner called up on my behalf and they said they received information the other day that meant they could go ahead without the uc50. My partner asked what the information was and who sent it but they wouldn't tell us. My partner asked to reschedule to be able to fill in the uc50 and send it back. And they said they would send me out another appointment for end of July beginning of August. Is there anyway to find out what information and from where they received it. I'm a bit scared to be honest.