It is as much in their interest as anyone else's to have a transparent investigation as failure to do so would potentially lead to a ban on air travel to/from Iran on the basis that it would be potentially dangerous.
The sanctions probably prevent Boeing from getting anywhere near it as much as the Iranians don't want them there... Norwegian still have a 737 Max stuck in Iran for the same reason (it had to make an emergency landing following a technical fault and they've not been allowed to get the parts in to fix it, the grounding of type notwithstanding).
They have released initial findings to the interested parties, as per their legal obligations, and I suspect that the Canadian government will eventually get involved given that the majority of passengers were Canadian citizens.
One thing that I find slightly odd, is that the flight to Tehran would have landed at about 1am local time, which would been about the same time as the missile launches... Surely anyone with half a brain would close their airspace before throwing ballistic missiles around
Also, Ukrainian Airlines no longer fly to Tehran...