For binary responses, you will still get a nonzero proportion due to interaction even when you do not fit the interaction. This is because the model is non-linear; the effect attributable to interaction is on the additive difference scale, but the model is being fit on the log or logistic scale. This is a known and derivable quality of Causal mediation analyses, but it is not clear how you would actually interpret when you don't fit the interaction explicitly.
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