That's a bummer that it doesn't work on iOS with 18 words... thanks for the headsup (I have edited my earlier post for anyone stumbling across this thread in the future).
I was honestly expecting it to say the same thing when I tried it on Android, but it worked perfectly with 18 words...
That's weird. I would have expected exactly the same functionality on both an iPhone and android. What happens if you create an 18 seed word wallet on android, then try to recover it on an iPhone? If my android phone broke I would expect to be able to chose an iPhone replacement and use the same 18 word bread wallet seed in it.