How do I change the bound constraints of the estimate function of the regARIMA class?

I am trying to do a regression on a time series for which I have specified an ARIMA model with some AR lags using the regARIMA function in the econometrics toolbox.

Subsequently I add some independent variables and estimate this model with the estimate function. I have done lots of regression in this way, but this time I am running into an upper bound constraint on the constant of the regression model.

The solver for estimate is the fmincon function (with some special settings such as the sqp algorithm) which apparently has an upper bound constraint of 10 as default for the model constant. This appears very strange because lots of models have a constant above 10 of course. When I divide the Y-variable by 10 everything goes well and the function finds the corrects solution for the constant of 1.3 (so actually 13 when not divided by 10).

My question is twofold: 1. Why on earth do the standard constraint settings enforce a model constant below 10? 2. How do I change the bound settings when using estimate of the regARIMA class? I cannot find this in the documentation, when I use optimoptions or optimset, the bound constraints apparently are not one of the choices I can make.

Many thanks in advance!