Yes, I have dealt with a similar issue, but with articles instead of timelines (though I have found that adding an event causes the timeline to go into view mode instead of staying in edit mode but that's a different issue entirely). And before anyone says that I had a second tab of the article open and edited in that one by accident, I don't have a second tab open. I only have one edit version open and one public presentation view mode open to view the article as my players or readers would see it. It can easily be reproduced with similar steps as above. I have tried to report this before, but was told that it can't be reproduced even though this has happened at least 3 times this Summer Camp alone, and a few more times before that.  

create article

type a bunch of text

wait for green checkmark

add a prompt box (usually from the header section) and add text to that

navigate away from article

go back to article to see your text is missing

I do wait for the green checkmark to appear, but that sometimes doesn't work. I have to reload the public presentation mode to make sure changes are actually saved before I can even attempt to navigate away from article so I don't lose hours of hard work.   I feel like this is something to do with the save feature or something similar.   Example of how I view and type up my articles: