When you enter a negative dimension value in a Family Table, Creo Parametric interprets the negative value as a flip of the direction each time the feature is regenerated.

This is because the dimension value is considered to be "relative" by default. To avoid this, you must change the column to be considered as "absolute" values. You can switch the column between relative and absolute by selecting a cell in the column and choosing Tools > Absolute / Relative Dimension Values in the Family Table dialog box. The dimension name at the top of the column will be preceded by a dollar sign ($) when the column is absolute.