I see you’re using extra functionality to display a posts list inside a (static) page, with the posts incorrectly displaying the page’s excerpt instead of their owns. This may be caused by several factors, including some theme functionality.
Since I don’t have a site configured with your exact setup, I cannot test for this myself. So please check if the posts list displays the correct posts excerpts when (temporarily) having one of the default Twenty themes active.
If they work correctly with a Twenty theme, reactivate Mantra and check if setting the theme’s two excerpt-related options to the “full post” option resolves this.