I first did the "Smelt Everything" achievement, and some days later I ran through the achievement list and noticed that I hadn't gotten the "Delicious Fish" achievement, even though I'd smelted loads of fish (of both types) and eaten the cooked fish.

To get the achievement I had to manually place the fish into the furnace, and pick it up, before eating it. It didn't work when the fish was automagically placed into the furnace using a hopper from a chest, and removed using a hopper into the chest, and then eaten afterwards.

This is not related to MCPE-10840, as I got loads of other achievements before and after the already mentioned issues.