The effect size for robust RM ANOVA is not available because it is not yet implemented in
WRS2 package, which
statsExpressions relies on.
You are right, the degrees of freedom for
ggwithinstats are different because they run different kinds of tests; the former ANOVA, while the latter a repeated measures ANOVA.
Here is a good introductory tutorial on
dfs are computed for RM ANOVA: