I’ve tried replicating the issue you’re having but was unable to.
WordPress 5.5.1 with wp-show-posts-pro-1.0.0-beta.4.
Perhaps you may have to update the beta version you use. You might also want to consider updating WordPress if it isn’t yet. I believe it fixed the color saving in WP 5.5
This may also be a simple caching or plugin conflict issue.
Can you try clearing your WP & browser cache to see if these issues still happen? Also, can you try disabling plugins one by one to check which causes it?
Let us know of your findings. Thanks.