Plane on Canvas:

Plane Shape downloaded as PNG with transparent Background:

Hi @Charlene, this is definitely unusual and not intended. A few questions to help us understand what might be happening here:

• Could you try opening then exporting your document in a private or incognito window to help us narrow down the potential cause? Could you also try connecting to a different network or disconnecting from your VPN (if applicable)? Sometimes account performance issues can be related to internet security/speed or browser issues. These steps will help us eliminate that possibility. For help with opening an incognito window, please see this community post (in English). 
• Is this the only document this is happening with? 
• Is downloading the document as a PNG with a transparent background the only background setting this happens with? 
• When you say that a PNG is the only form in which you can download this document, do you mean that that’s the only option you can pick within Lucidchart? Or that this is the only form that will work for your needs after exporting?