You need to sort the Estimates data set by the label and then MIANALYZE BY LABEL to combine the results. You should also use the DATA= input data set since it is not a data set of parameter estimates.
Add the following just after the GENMOD step
proc sort data=pairwise_estimates;;
by label _imputation_;
run;
proc mianalyze data=pairwise_estimates;;
by label;
modeleffects LBetaEstimate;
stderr stderr;
run;
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