Found a problem with your mod (possibly). Yesterday I had a bit of a stupid moment, got confused with my H-Shifter and threw the car into 1st at high speed. Naturally the engine imploded so I went to Fleetari to buy new parts. I did that, all went well, put the parts into the backpack, got home (using teleport) and tried to empty them on the worktop to assemble the engine. The parts just didn't appear at all. They left the backpack never to be seen again.
I have another mod that allows me to teleport the parts to me, so I tried that and nothing happened. So something REALLY bad happened in the process. I didn't save the game, came on it to test again and the same thing happened. :/
EDIT: Okay, this is unusual, I just loaded the same save twice to replicate the bug and both times now it has worked fine. I have a feeling that it's based upon how many parts you get. The past 2 times I have only had 2 items purchased since I haven't taken apart the engine, but I think that 3 or more and the bug occurs. I will strip the engine now and report back.
EDIT 2: Okay, after some testing I now know the exact problem! This bug occurs when you press E to put multiple items in your backpack and teleport. I think this is only true of engine parts since I have not had this issue before. The way around this issue is to click on the item to hold it first, THEN press E to put it into your backpack. Then if you teleport, the items spawn properly! Hope you can fix this soon. Thanks.
EDIT 3: Okay, so I started a new save just as a test, turns out ONLY the parts you buy at Fleetari's have this bug... If you just put the standard parts you get at the beginning, it works fine. Hope that's enough details.
P.S. Apologies for the long comment.