I’m not sure if this is a ‘bug’.
For example, start a skirmish and set up a market and some granaries. Sell all your wood so that you can’t afford husbandry or plow. Now select your granaries and prequeue husbandry and plow. Both will be prequeued in the same building despite having multiple granaries selected. I would have assumed that the prequeued techs would be distributed evenly amongst the selected buildings, just as non-prequeued techs are, and just as units are.