Yes - I've seen this in my tenant too. It seems to have stabilised this week.
At the same time as I was experiencing the disappearing publish button, the 'Undo' button had ceased to function as well .
I'm not sure if the two issues were related (both objects are a part of the page editing tools).
As of today, the Undo button is functioning again and I think (fingers crossed) the publish/republish button is also behaving itself.
My Admin had reported the undo button problem to Microsoft Support, so I expect a fix has been applied to my tenant. You might see if you can get your Admin to check to see if this issue has been included in the Office 365 Admin service support area.
@Harsh Damania I've experienced this a lot this week. I simply save as draft. Then "edit" it again. If no publishing button, I refresh the page and it is there. Frustrating, but it has worked every time.