Actually Bringthechallenge just reminded me of something - I was playing around with the Khergits, and there fifth tier archer isn't actually accessible. At least, not on my installation. Every other troop upgrade path works fine, except this.
Now that Bringthechallenge has also mentioned it, this may be a Floris bug. Just goes to show how little the Khergits are played ^__^
EDIT: Also posted in support