Are you or your parents by any chance Caribbean / African / Indian heritage? Don't feel you have to answer, but that seems to be the core of the issue.
You can read some discussion in Parliament about this here:
https://questions-statements.parliament.uk/written-questions/detail/2020-01-14/3136
https://assets.publishing.service.g...33106/20200924-FOI09850-Response-REDACTED.pdf
As well as the recruiting page and leaflet here:
https://apply.army.mod.uk/how-to-join/joining-process/soldier-recruitment-steps/soldier-assessment#
https://rppcdn.azureedge.net/-/media/files/joining-instructions/sickle_cell_trait.pdf
Basically, this looks like
a reaction to two soldiers who died in training late 2019. Sickle cell diseases come from a genetically inherited trait which is most prevalent in Africa, India and less so in the Arabian peninsula, and it can cause a variety of problems (as well as one benefit, malaria resistance). The fourth link above says it is not disqualifying to service, although the criteria
@Admin linked in the Medical forum says that it is disqualifying for the Parachute Regiment. You'll have to ask the question, depending on your test results.
Sickle cell tests are commonly done at birth by the NHS, but aren't compulsory, so it's possible to have it and be unaware. But it seems that the Army has instituted a requirement as of last year to test certain applicants for the genetic trait. It's not spelled out, but this almost certainly means anyone with particular BAME heritages as above, because those populations account for over 80% of people with the condition. It's probably not spelled out because that is quite an embarrassing policy, given the Army's explicit drive to recruit BAME individuals.
What you can do. As you can read in that second link above, the best option you have is to ask your GP to check if your health records have a SCT test result. If you don't have one, you'll have to get one - I presume the AC told you how (links above say they are providing them free). If you do have one and it's negative, ask your recruiter why you were treated differently at AC. Send that second link to them when you ask, as it's a formal reply in parliament saying
that shouldn't happen. I'd be interested to hear their answer, because if you already have a negative SCT test, they are clearly in the wrong.