This has happen a few times.  I work on a Confluence page come to lucidchart to create a diagram then switch back to Confluence and use the integration tool ( not a picture ) to insert the chart.

Some times it is really hard to find the chart and I work around it by putting the file in a folder which makes it visible.  But I have checked every file and use the search tool so I know the chart is just not visible on Confluence most of the time. 

The second behavior is where the chart is completely lost.  I went to Confluence the chart is not there.  Then I come to lucid chart and the chart is there I have named and saved the chart at this point.  

I then go to the file browser and the chart is not there.  If I try to go back to the page that has the chart with the diagram now the page is not found.  So I have no option but to create the diagram again. 

On the positive side this happens most often with short quick diagrams which makes me think maybe you have some issue with an eventually consistent storage device or a race condition.