My secondment position has been made permanent without my knowledge

I took on a Secondment at a different office while our organisation was merging with another. After the merge, I was given a contract so that we all had contracts under the new organisation name. My secondment isn't due to end until October but I was asked to sign the contract in April. It was a permenant contract but I assumed that that was just to ensure that I was still protected with my fulltime hours and permenant role within the organisation. I double checked today to ask what the exact date my secondment ends. And it appears that my position was made permenant at the end of March when we merged in April.

I hadn't been informed of the secondment coming to an end but I have signed the permenant contract. Where do I stand with this because I would like to end my secondment and return to my previous role. My previous manager also still believed that I was seconded so I'm unsure where I stand.