Negative altitude during mission flight

If I lift the drone when it is not armed the altitude results to be more or less correct (50cm of error) but once I start a mission (in my case it has to reach 2 meters of altitude) the altitude is wholly messed up and it’s going under zero, resulting on an endless take off because the drone never reaches the requested altitude ( Video example).
In the parameters, I set EK3_SRC1_POSZ= 1 (Baro)
I use a PX4 3DR with this barometer: MS5611
Arducopter V4.3.4 Official
Do you have any idea what could be the reason?

ps. I’m not using GPS, I’m using intel real sense camera for indoor flight
pps. In the video there is the warning “bad OptFlow health”: I recently installed an optical flow but before this flight I removed it, btw I don’t think it is the reason for the strange altitude behavior