This isn’t a bug report, etc, just something interesting, a FYI: No fix required. It could also be a “me” thing.
“L” mode doesn’t work in design mode for me but I found out how to get there.
Here is a way to get “L” mode in design mode if you want it:
Open your puzzle that is in design mode, open up a new tab and select any already made puzzle, then toggle to “L” mode. Then delete that tab. That brings you back into your puzzle in design mode. Select natural state and the puzzle will be in “L” mode. You are however stuck n “L” mode, toggle doesn’t work, but exiting and entering the puzzle again brings you back to regular mode.