Capital gains tax annual allowance

I sold a residential property in FY 23/24. My gain was below the £6000 threshold for that year, and I had no other capital gains. I completed a reporting return and was told there was £0 CGT to pay. I included the sale in my tax return, using form SA108. My tax calculation just received says I owe £830 CGT on the gain. Why would this be? Additionally, the tax has been calculated at 18%, which is the effective rate from Oct 2024 even though the sale and reporting took place in 2023, when the rate would have been 12%. What is going on?

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Not HMRC...cant comment on the CGT due (other than appears something filled out wrong on the return as if within the annual allowance when did CGT return and no other gains then it should still be within it on the SA return)...talk to HMRC...but CGT rate has been 18% up to unused BR band since 2017, not sure where the 12% comes from?...consequently 2023 was 18% also.