Sorry, I must have missed that part the first time.
That's definitely odd. I just tested on my side and I could enter any value for the delay on a Fade animation...
Would you mind sending me a PM with a link to the .pixate file? You can upload it to Dropbox or Google Drive. If you're using the web app, just export the prototype and do the same.
I'd like to take a look and see what's going on. Please include the name of the layer where this is happening.