My guess is that for some reason it things you are trying to use a non-english language, using something about how your OS and/or browser is configured. Still, this is strange... It did not happen to me, but it did happen to at least two users that really doesn't make sense. I'm going to tag it as a bug.
Hyphens are excluded from tag autocomplete. ion-thruster appears as you type ion-t or iont -- hyphens are ignored both ways.
Also, tag autocomplete is a full text substring match, not a prefix search, by design.