I've tried to simplify the code as much as possible with this example:
If I create a chart with a shared tooltip, I can switch back and forth between shared and unshared tooltips without issue. If I create a chart with an unshared tooltip and try to set shared = true in the options, then redraw the chart, the tooltip will not display on hover. It will display on click, but even showOn = "mouseenter" will not help.
I've compared the options objects between the two charts in the example and they are essentially identical with a few spots where the series tooltip is still false or the font is set. The commented out code is meant to show that fixing these differences does not make this work.
At this point, I'm assuming that creating the chart with shared = true does something that shared = false does not, e.g. registering events for the shared tooltip, but I can't figure out what it is. Is there something that needs to be done in addition to setting shared = true or is this a bug that needs to be reported?