Not sure if I diagnosed this correctly, but LiveSplit seems to retain a time entry in a subsplit container after resetting, it'll look like this:

There you can see I reset on one of the subsplits in the W1 group.

That time will stay there into the next run until I get to that spot in the run, or, go to "edit splits" and press cancel. Then it will erase. Going to "edit splits" and clicking okay doesn't reset it.

Is this a bug? Feature? And can I prevent it?

What are you comparing against?

It shows up on any comparison. Side note though, it disappears on using the comparison-change hotkey as well. Maybe I'll just use that...