Unfortunately, I find that this is not repeatable when I construct all-new linework.
The situation is that I have an arc between elements--two curves. I want to continue using one curve but I want to change the other parent. The parent that I can't select is now a member of a complex chain. I wondered if that was contributing to the problem. I actually have two such curves, both using the same parent that I no longer want to use.
To test whether I was right, I constructed some linework out in space, set up a curve between some of these elements, and then created a complex element to include one of the parent elements, and I was able to select it just fine, so I know it's not that.
The fillets I want to change the parent on are "w departure" and "w approach". I'll be uploading the file presently by the secure upload instructions.