Newsletter block becomes invisible when in edit mode

I have had a response from Squarespace:

Thank you for reporting the behavior with the content of your newsletter not displaying while editing the page. After further investigation, I can confirm that this is due to an issue on our end. The issue will occur if the newsletter has the Background option toggled on. 
 
We’ve noted this for our Design and Engineering teams to review, and hopefully this will be addressed shortly.
 
Please keep in mind that there are many moving parts we need to address before any changes are made, and we can't guarantee changes to this behavior in the immediate future. That said, all feedback is reviewed and your continued input is always encouraged.

For the time being, you may want to consider disabling the Background tweak within the Design tab of the Newsletter Block. This way the content of it will be visible while editing the page:

https://share.squarespace.com/E0uXXPoz 
 
I want to set the expectation that this is an alternative solution you can use temporarily, rather than a permanent fix for the exact issue.
 
Thank you again for reporting this. If you come across any other unusual behavior in the future, please let us know.

After following the above advice and going into my site styles and changing the 'newsletter block' colours, all is fine now. Although not ideal, the solution provided is a workaround.

Thought I would let other users know just in case anybody else has this issue in the future.

Regards,

Becky