R U sure you didn't accidentally delete it ?
When you created the 6th Part, was it by any chance created by "Save as" from the 5th Part? If so, maybe the references got messed up.
While I won't deny the existence of "The SolidWorks Factor", sometimes it gets too much credit.
You had to hide, then un-hide the fifth part to show it? My first thought was TAB was held down when hovering over either of the fifth parts.
I don't suspect in-context development would be the culprit either. However how stable is your part template? Have you ever seen this happen before?
I have no real answer to your particular problem, I would try to re-create the problem by inserting a new part into your assembly like you did #6 and see if you can re-create the issue. It has helped me in the past to figure out if I muffed something up somewhere along the line.
It happened to me some time before but I didn't pay much attention and investigate more into the issue
Esc key sometimes functions as a delete key which I thought I tried to cancel what I was doing, but ended up deleting a component in the assembly
The fifth part was made yesterday and the sixth part today. Last night I saved and shutdown my station. Solidworks glitched there were no missed or unplanned key strokes