At the top left there is a indication of when office online is saving your work
This can be stuck in saving for lengthy time frames and if you close your tab before it says Saved you lose everything you entered in that time frame with no prompt to warn you.
There should you be a prompt to let you wait for it to finish saving your changes before closing out and losing them.
I had it to where it was taking an exceedingly long time during a training so I was consistently losing my work in front of the customer without knowing why at first.