Why is this deleted title transition still visible?

That's a weird one. I've not seen that with Basic Title. Try selecting everything in your project timeline (Cmd A) and copying, then create a new project with the specs you want and paste (Cmd V). Sometimes subtle project corruption can be fixed by copying and pasting into a new project.

If that doesn't work you can try deleting FCP preferences. Hold Option and Command while launching FCP and choose Delete Preferences. It will open the default Untitled library. Re-open the library you were working on, set the Preferences back to the way you like them (disable background rendering, reset Import to Leave Files in Place, etc.) and see if the issue has been resolved.

In addition to the good advice by terryb: you could also try deleting all render files: select the library, and do File->Delete Generated Library Files, and check All Render Files.

Also, if you haven't already, turn off background render in Settings->Playback. Note that this will be turned on again every time you delete the preferences.

Great! Glad it worked. Since you're new to FCP, you would probably really benefit from the go-to, free tutorials recommended here. The time spent watching will be paid back 10x.

Steve at Ripple Training, 60 minute FCP tutorial overview https://www.youtube.com/watch?v=HqUP7Zgeuck

Izzy Video tutorials. Older so the UI looks different but the concepts are still correct. https://youtu.be/_jvyrnQpR1E?list=PLp5SAgbM8S87n1qvv34-PAU2zMHLlXyfb