When I do "CTRL+Z" I can't see the onion of the previous frame

Hello! I’ve noticed… When I ever I “Undo” anything, the onion of the previous frame goes away, it’s pretty annoying (I look at the timeline and the onion is still there, but in the camera view it isn’t visible)

I’ve moved over from Flash and I’m trying to get the hang of things, does anyone know how to make the onion skin stay?

edit: I’m using Toon Boom Animate (7)

What specifically are you undoing when this happens?
Which version of Mac or Windows?

I’ve got the same problem and this is in the Mac version.

You have to click on the frame (cel) in the timeline and the onion skin becomes visible again.

It may be an issue of losing the mouse’s focus so the previous posters is good.