The TOC in the Cloud sidebar is collapsed by default. In the Server version you can set the level of nesting for the TOC. How do I expand the TOC to 3 levels in Cloud?
I believe this is the same question as here: https://community.atlassian.com/t5/Confluence-questions/How-to-automatically-expand-show-all-child-pages-in-the-sidebar/qaq-p/333429
In that case, it has already been answered :)
Oh, my apologies. In that case I think it is indeed not an option for Confluence Cloud. You could definitely raise this as a support issue, but unfortunately I think it will take quite some time to resolve, in the best case scenario. Atlassian is currently swamped with the roll-out of the new editor and the bugs resulting from that.