Hey Daniel,
If you want a quick and fast solution, as it sounds like your project is holding onto an old reference when you tried to test the landscape grass stuff, would be to delete your Saved, Intermediate, and Config folders. Make a copy of these folders first before doing so, but once you have deleted these, re-open your project to see if that resolved the issue.
Honestly, I would need a way to reproduce this in a new blank project to get a better idea of what is exactly occurring, but it sounds like you definitely have a corrupted reference or asset.
What you can do to help you troubleshoot your issue is to go to the Help > Console Variables option. It will create a searchable HTML file so you can narrow down helpful variables which might help you locate the source of the problem.
Let me know if you have further questions.
Cheers,
Andrew Hurley