That’s a borderline case between bug and feature. Paperpile allows you to write LaTeX code in the title and book title field. That makes it easy for example to write the chemical structure of water as H$_2$O and it will be displayed correctly styled in Paperpile.
We also allow to use “{” and “}” to mark words where the capitalization should be preserved. If you really need a “{” or “}” character in your title you need to escape it with a backslash like “\{” or “\}”. On BibTeX export, Paperpile will do this escaping automatically for any field except the “title” or “book title” field. Any “{” or “}” in the abstract field will get escaped automatically, while for the title field we assume that the user has put them there on purpose.
In your case it seems that the number of curly brackets is unbalanced. In that case it is a good idea to show a warning.