Forms Not Redirecting to Thank You Page & Appending #wpcf7 to URL

For about 4-5 months now, my “Thank You” pages are no longer working with the Contact Form 7 plugin. Instead of https://www.example.com/contact/ redirecting users to https://www.example.com/thanks/, users are redirected to a URL like https://www.example.com/contact/#wpcf7-f2045-p30-o1. I’ve updated WordPress, my theme, and all plugins on my development site and it’s still broken. For this reason, I haven’t updated the Contact Form 7 Plugin since mid 2016. I’m afraid it’s going to be broken on my live site as well.

Of all the support threads that I’ve read about this issue, the plugin author just says things like “use the default” them and such, which simply isn’t practical.

How can this be fixed? It’s been pretty disappointing.