MySql MBRCONTAINS not accurate
MySQL's result seems correct... MBRCONTAINS() tests whether the Minimum Bounding Rectangle of one object contains the other object.

Does the smallest possible rectangle you can draw around the polygon... contain those points?  Visually, it certainly appears that it does.

To test whether the polygon, itself (not the MBR) contains the point, the function you want is ST_Contains(), which was introduced in MySQL 5.6.

http://dev.mysql.com/doc/refman/5.6/en/spatial-relation-functions-object-shapes.html