Thanks for letting us know about this.
We tested the custom time format G:i A in WordPress General Settings on our side, and it’s showing up correctly in the EventPrime time drop-down. Here’s a quick screencast screencast.mp4 for reference.
This issue may have been fixed in one of our recent releases, so please make sure you’re using the latest version of EventPrime. Could you share a screenshot of what it looks like on your site? That will help us pinpoint the difference and get it sorted quickly.