As far as I know TicksBetween has never been a property of axes.
You can set the location of the minor ticks by changing the MinorTickValues of the numeric ruler: ax.XAxis.MinorTickValues = movmean(ax.XAxis.TickValues, 2, 'EndPoints', 'discard');
ax.XAxis.MinorTick = ' on';
will give you exactly one minor tick between each major tick.
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