After updating to WordPress 5.5 side bar is missing on new posts

I just updated to WordPress 5.5, after which I made new post and the sidebar is missing on that post. The sidebar is still there on my previous posts and on the other pages.
I attempted to do another one and had the same result.
I went into to Customize > Widgets and the sidebar thing is no longer there for that post. I changed to another post and the sidebar thing is there.