It refers the your last main residence you owned, and lived in, and didnt dispose of, not the one you currently live in and dont own.
You owned and lived in another property before moving in with your husband that you didnt dispose of so that is the property for you that is relevant, you retained your previously owned main residence.
You can own multiple properties and not fall foul of the SDLT higher rate, but they have to be properties you have either never used as your main residence or ones that have stopped being your main residence as you purchased another property to be your main residence.
So you will pay the extra this time but if you move again after you jointly buy the new property then the current one causing the issue will no longer as it is not the last main residence you owned and lived in.
If they didn't do it like this people could just move into an Airbnb and rent out their main residence and say we'll I didn't live in it at the time I bought a new property.
I don't agree with it, I fell foul in similar circumstances to you in 2016 and it cost an extra £16k in SLDT, but it is what it is sadly.