As much as I'd love to help with that, I'm afraid I haven't any idea about this, nor about whom to ask. Duolingo runs hundreds of A/B tests - literally - and I know they're constantly experimenting with our experience points. Though I can't be sure this is what happening, that's my best guess. Or it could just be the website acting up... or something else.

You're on the right track! I was very into fantasy as a young teenager, and wanted an elfish-sounding name - and I thought for the longest time that I'd created devalanteriel myself, only to find out later that the name appears in a book by Katharine Kerr which I know I was reading around that time, so apparently it's not one of my own invention after all. He's only mentioned once, during a fierce battle in which he lay mortally wounded. Alas, his sacrifice shan't have been in vain - I'll endeavour to remember him fondly forever.