Applied for Unmarried partner visa (2,5y) granted D-Marriage/CP 6 months

Hi everyone,
My partner and I recently applied for my unmarried partner visa to return together to UK from NZ (he is a Brit citizen). We provided all necessary docs proving we’ve lived in stable relationship for 2 y+, meet financial requirements, but marked we’re not getting married in the near future.
In around two months I received my passport back with the fiancé’s 6 months visa in it. No accompanying letter followed.
To say it’s a big plan changer is to say nothing. Mainly because now we have to organise a wedding, that we were not planning AND go through the same application process to switch to spouse visa in 6months time. To be honest in the COVID situation the spouse visa+ IHS fees are really tough for us.
Any ideas why this might have happened and is these a chance to appeal or even cancel and get a refund?
Thanks all and regards

Ask them to correct it.