This paper is an addendum to a previous paper of mine, which discussed modified XVERT mixture screening designs (MXMSDs) for mixture constraint regions defined by lower and upper bounds on the component proportions. MXMSDs consist of a center point and a subset of the extreme vertices derived from a classical two-level screening design (e.g., Plackett-Burman or fractional factorial) using a modified XVERT algorithm.The previous paper showed that the properties of MXMSDs can be nearly as good as or equal to the properties of designs in which the subset of extreme vertices is selected using optimal experimental design software. In this addendum, it is shown that the properties of MXMSDs can also be considerably worse than those of optimal designs for some mixture constraint regions. Simple functions of the component lower and upper bounds provide a basis for determining whether or not MXMSDs will have properties close to those of optimal experimental designs. Two versions of a nine-component herbicide example are used to illustrate the discussion.