Bug: Undercroft Buy/sell

Its not a bug, unfortunately its a feature. All the creature research items are labelled as valuables and will be sold if you use "Sell All". So you have to buy them back (use merchant buyback feature, last section on the right), or sell all your valuables one by one. As long as you haven't transitioned to a new area, you can buy back from any merchant (so if you sell at the blacksmith in your homebase, you can buy from the merchant).

It is best to use your homebase as your main selling area, and turn in creature research before hitting the vendor. The creature research is not necessary to beating the game, it just gives you damage boosts and some background lore to read on the creatures. And as long as you haven't completed the research, the creatures will continue to drop research items. It really only hurts you if you sell rare creature items like some Revenant research items, since there aren't many in the game.

As for the requitistion items, until you turn them in, more will drop. It is only a problem if you push the story forward and the enemies that drop them no longer spawn, like for the Ferelden Locks requisition. However, these quests are just used to generate power and influence, and each area has its own set of them, so if you can't fill a requisition and it blocks you from getting more requisitions in an area, you can go to a new zone and start doing them there. These "fetch" quests are something you will eventually avoid doing, as you will get more power than you can spend, and influence can only be earned to level 20, and the requisitions are a slow way to earn it.