Mini-map shows X's rather than chevrons for nearby people >1020m up

Repro:

• log in with an alt and teleport both to 2000m (or anything above the coarse location maximum of 1020m)

• if both are at the same height the "same height" regular dot is visible on the minimap

• have the alt fly upward
  => the alt's dot turns into an X even though avatar location data is available through its LLCharacter instance