Versions tested: 2020.1.0b16.4138 + 8.x.x./release (32749aa at the moment of checking)
Reproduces both on Windows and MacOS

Steps to reproduce:
1) Create a new VFX graph, open it.
2) Make several edits (add a new node, connect it, move around).
3) Try to press undo a few times.
4) Notice how it only undoes the very last action in the stack, it tries to undo the next one but its quickly getting reverted

Note: If you press ctrl-z and hold, it will force to undo all actions after the first one.