HTML not saving in gutenberg

Hello, I am working on a new not yet published website, and have a page where I need collapsable content.
so I found this plugin and started using it, the problem started after I saved my progress for the day updated the page and came back the next morning, and the HTML code between the expand was blank. all the text and images everything I wrote was gone.

I tested this again today, by just opening the editor in a new tab after saving the changes and it does this again.

the weird thing is, some lines stay while others don’t. like lines I wrote manually but not all of them.

as the topic suggests, I am using Gutenberg to edit the site.

this is on WordPress 5.6 with everything up to date. is there a way to fix this? or a setting I need to change?
thanks!