Normal behaviour:

With default controls, holding a stack of items and dragging while holding right click drops one item into each inventory slot you drag across. Similarly using left click would spread the stack over every slot you drag across. Finally in creative mode using middle click would create a stack of that item into each slot you drag across.

Problem:

If you assign one of the "gameplay" controls, those which default to the 3 mouse buttons, away from their defaults, these abilities do not follow.

This could be considered a feature since for left and right click the control options do not directly relate to the inventory actions. Additionally, for these two the ability is still there on the left and right click buttons so no functionality is lost.

I would suggest that for Pick Block and therefore middle click this ability should be transferred because the ability does not remain attached to middle click (ie, dragging while holding middle mouse button does not do anything, neither does holding the reassigned key or holding both at the same time.)

I came across this issue as my middle mouse button hardly works and hence I use the R key for Pick Block, however I then realised that I could not duplicate blocks. By reassiging the Pick Block control, the functionality is lost.