Crimea was a referendum and Eastern Ukraine separatist rebels. Extent of russian involvement aside, there is a difference between that and an all-out invasion.
If they occupy Kyiv, there would also be all kinds of "referendums" and "local militias" with predictable results. It is not like anyone remotely pro-Ukrainian will stay and vote under watchful eye of friendly uniformed people (that don't wear any identifying insignia).
What on earth has Kosovo got to do with this now then? Is it whataboutism? What is it?
There have not been any free/observed/internationally recognized referendums on Crimea. Most importantly, the referendum wasn't with blessing from Kyiv.
There was an annexaction which was partially done with military means and partially through a mock referendum. Both of those are blatant attacks on Ukrainian soverieignty.
Kosovo has 100% to do with this, friend. In case you are genuinely curious: The "West" has decided that Kosovo suddenly must be independent, and they bombed the shit out of Serbia (6mil pop, tiny) on a pretext that somehow this just HAS to be a separate entity. They didn't even bother with, as you say, a "mock referendum"!! Now the same people come lecturing about borders and sovereignty, and referendums, sounds cheap as shit.
They occupied the territory first and then conducted the "referendum" to make it "legal". Don't buy this horsesht.