That's odd. The red arrows don't show up either. I followed the exact procedure that you described and the text shows up in both the media browser and the timeline. Are you certain that you are not by chance dragging the original photo without text into the timeline?
What happens if you click on the project name under Project Media in the side bar, and drag the text photo from the media browser into the timeline? What happens if you export the text photo from the Photos app to your desktop, and from there drag it into the iMovie timeline?
If none of the above work, try deleting preferences and see if that cures the issue. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich