Nothing To Undo - Bug Possible?

Yesterday, i decided to create a model in the WIP version just to familiarize myself with the upcoming version of Rhino.
What i encountered twice was that after a point the undo was not working and a message “nothing to undo” was showing.
Once i restarted and run it again i was able to work again but after some point the same thing happened.

I have the latest WIP version
My computer is a Macbook Pro 2015 but on Windows.

What is your memory setting for Undo for the V7 WIP?

In V7 WIP DocumentProperties > Rhino Options > General > Undo