Just had this happen me to a very poor sim who then had a money fruit drop into his inventory while I was reading this post. I went back to the dumpster to look for other stuff and it disappeared. I'd already lost a mulberry, a cherry and a grow fruit. I took it out of my inventory to make sure it wasn't a dream and then put it back into my inventory, maybe in hopes it would solidify it. I wonder... is it possible that these things are falling out of the sims inventory into the dumpster like they are losing the items when they go diving?
I am also having these problems. My sim won the money tree seed at the dump in Evergreen Harbor. I was very happy! But when she returned to her home in Glimmerbrook, the seed from the inventory was gone. I was sad, and the worst thing happened 2x.
A similar thing is happening to me. Except, when I knit anything, it doesn't appear in my inventory. So it's kind of useless as I am unable to sell the items, so then, what's the point?