Once set (or not) this option does not have effect if you try to change it dynamically. One possible solution is to define the multiselect when you set the grid then use heideCol('cb') (where cb is the name of checkbox column – see docs) and under some condition showCol('cb') but I think this will not do the job for you since the selection remain. Also you can try to play with multiboxonly set to true in the case.
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