Cannot scroll sideways in a matrix table in mobile layout.

I just encountered the same issue. It seems that it is becase the row header panes are frozen by defult. In Power BI Desktop you can unfreeze the headers by left clicking the matrix in desktop view. Then when you switch to mobile view the issue is fixed and you can now scroll left-to-right. So, the issue is not that you cannot scoll, it is scrolling. You just can't tell because the row header panes are frozen. However, this is still a major issue as you cannot save it in the unfrozen state. Therefore, if you have a wide header it seems as if you cannot scroll in the mobile version of the report. If the Power BI team could make a togglable option in the visuals setting to have the panes frozen or not, It would solve the issue...