If you don't have a dehumidifier, all of those big bags of mostly water that inhabit the building will expel some of that water with every breath, and going to increase the humidity fairly noticeably.
I'm also going to guess that the MVHR is also not 100% efficient. It's either going to create heat to completely cool down the incoming air (hey, look, an AC, complete with dehumidification), or the incoming air is going to average out with the out going air, creating an increase in temperature.
Plus, all of those bags of mostly water are going to be expelling heat, too.
You don't have a dehumidifier. One of the biggest benefits of fabric first is the reduction in complexity.