Paste remembers layers bug with isolation mode
If you choose the option Paste Remembers Layers and copy elements when in Isolation Mode and paste these after exiting Isolation Mode, they appear on a new layer named "Isolation Mode". They should appear on their parent layer instead.
Egor Chistyakov commented
Turning option off is not a solution, but workaround.
Well, if it is intended though, I can understand the logic about it, but I can't imagine the purpose of it, so I think it's still a bug.
Turn off "Paste Remembers Layers" before pasting. Problem solved.