Interesting. I just checked real quick to see and I can swap inventory 0 just fine in kb mode.
Are you sure there wasn't some other cause? Like trying to swap between a large and small part (actually, I'm pretty sure you're not allowed to swap different-sized parts, so maybe that wasn't it).
This appears to be some issue with your machine/keyboard setup, though it would be nice to figure out what. We'll see if anyone else reports it, and for now I'll leave this thread open here.
Some questions that might help narrow it down in the future: Are you using a US keyboard? English OS? Windows? (and version)
Some obvious workarounds: Cycle the inventory, if possible (as you did), or if you have exactly 10 items and cycling isn't possible, you can also press 't', 'm' or 'i' to re-sort the inventory and move items to different positions.