I have no idea where bug tracker is, that's why I decided to post this thing here.
1. Video proof of existing bug: http://www.youtube.com/watch?v=xcQ3tx45fzA
2. 1)Video proof of how it should work from official server (28 dec 2005): https://youtu.be/FeFZY-W-Z3w?t=1m12s
2)WoWwiki patch chronology regarding "Freezing trap": http://www.wowwiki.com/Freezing_Trap, esp. these lines: Patch 2.3.2 (2008-01-08): Freezing Trap is no longer limited to one target at a time.
Patch 2.3.0 (13-Nov-2007): Fixed a bug in which Hunters were able to use Readiness in conjunction with Freezing Trap on two different targets.
So prior to 2.3.0 that was considered a bug (NOTE: however it was doable) and after continious whining after 2.3.0 it was brought back in 2.3.2.
3. Bug importance: 8/10, since it's very useful in both PvP and PvE and includes core hunter abillities.