I’m sure I’ve discussed this before, but the behaviour is known (and somewhat deliberate) and is not viewed as a bug I think. As soon as the tempo varies, even a little, the os assumes a ‘new’ change and effectively blocks that undo, or possibly treats the tempo change as the last variation in the edit buffer. See if undo still works when you delete a pattern and then adjust tempo a fraction and then undo, whilst slaved to nothing!

It’s a pity that the behaviour is configured this way, but it’s been known by hq for a long time