Thank you, Junpeng, for such a fast response. Now it have become even more weird, bacause calling az.plots.plot_utils.calculate_point_estimate('mode', trace['tau'], bw=4.5) results into array([29.09679086]) – super close to the value returned by the scipy.stats.mode. Yet, plot_posterior shows 7.339. The only parameter which could have effect (bw parameter) does not really have effect. So I am now wondering what could be the reason plot_posterior is showing such a value of mode.