SX: ContinuedFloat not working

From https://tex.stackexchange.com/questions/442529/continuedfloat-not-working-sharelatex:

\documentclass{memoir} \newsubfloat{figure} \usepackage{subcaption}

If the counter is already defined, the code of \DeclareCaptionSubType assumes that this was done by a previous usage of \DeclareCaptionSubType, but this is not the case here.

So this assumption should not be made.