Location: Osteem Orbital (Dau G11).
The Wiki says that there are 4 prospecting missions/system. Yet, I did a sequence of 3 prospecting missions -- asked to look for (Carbolic, Aqueus, Ishik), respectively. At the end of the Ishik mission, I was advised that there was no further work in this system & sent to the next system in the mission tree. This suggests that through some bug, the tree has "lost" a mission?

Further evidence of this problem -- the mission log for the 2nd (Aqueus) is severely munged -- it contains a great deal of message data generated from the Ishik mission.

My concern is that, if I understand the wiki correctly, I can no longer complete the prospecting mission tree as there will be no way for me to get the 4th mission in Dau --> the best that I can now do is 119/120.

the key word here is 'average' not all systems will have 4 missions as some will have more and some less. You are missing one missions somewhere so revisit all the systems again and check with each faction to see where you missed one.

Thank you -- that meets my needs.

"Average of 4" is perfectly acceptable. I read the wiki entry entry as *exactly* 4. It is possible that the wiki could be clearer, but also possible that I made assumptions.