This topic is dedicated to documenting forum frontend debugging issues ie. Discourse not behaving as it should but nothing’s wrong in the backend.
The Event Calendar was not letting me post any new events. I ended up having to turn off a setting at the bottom of the event calendar settings titled “All topics in this category must have an event.”
This was strange because I did have an event in the topic/event I was trying to post, and I intentially selected that setting during plugin setup to ensure all topics were events. A plugin update likely messed this setting up somehow.