Should freezing a player actually remove the jetpack?

No, similar problem with setElementPosition, setElementModel and setPedAnimation the jetpack is removed but jetpack function (maybe just server side) still think a jetpack is being worn.

checked with v1.5.5-release-12363:
setElementFrozen removes jetpack but not the sound
setElementPosition removes jetpack but not the sound
setElementModel works without removing now

Another issue found:

1. Set model
2. Give jetpack
3. Set animation (jetpack will be gone, but sound won't)
4. Give jetpack
5. Remove jetpack (sound will be gone)
6. Set model
7. Your original animation which was interrupted by giving jetpack in step 4 is now playing again

Animations are not properly cleared with jetpack it seems.

PR submitted at https://github.com/multitheftauto/mtasa-blue/pull/229

Keep in mind the warp argument (true resets tasks, so it resets the jetpack as well). What I found however is peds and players do not share the same behavior. This should be unified in my opinion. Right now I can have a ped warping from A to B on a jetpack, while players can't. Problem is, if we enable Teleport for peds, it will not be backward compatible...

Also, exact same issues happened with setPedChoking. They are fixed in the PR.

PR merged: https://github.com/multitheftauto/mtasa-blue/pull/229