I contacted Squarespace chat support about this issue, which also just started occurring this month on my website as well. It appears that some custom coding on the design of my blog page template was hiding the blog post editing in the content pane.
I am able to edit blog post titles by adding /safe to the config URL once logged in (www.yourwebsite.com/config/safe). The support person insisted that the UI hasn't changed at all and that the custom code was the reason the editing options were missing from both the content section and the settings pane. I'm not sure what changed or if you have CSS but disabling the scripts during preview allowed me to save new blog post titles. Hope that works for you!