I seem to have run into a bug in a file I am working in, where the lasso selection is not showing up when I use it. In this file, when I click and drag to create a selection, the selection border will not show up until I deselect. Doing some testing, it seems like it is being caused by a specific color palette I made while specifically in indexed color mode. If I create a new blank file, regardless of the color mode, the lasso tool works correctly. If I create a new blank file in RGB mode and import the color palette I was working with, the lasso tool works correctly. If I create a new blank file in indexed mode and import the same color palette, the problem shows up again.
Note: I am able to get around this issue by switching to RGB mode.
What is your alpha color? If it’s the same as the lasso tool outline color, maybe Aseprite is mistakingly not drawing that color.