Without specifying a group or situation, they rules and procedures for seceding should be followed. If the process fails to deliver your wanted outcome then you have to abide to the rulings.
What is not ok is for a foreign body to interfere. Certainly not by invading said country and killing, torturing and whatnot. If secession is successful then that autonomous new country can join whatever other country at their hearts desire. But again, that other country is not to step in and force secession.
Now what if the plight is of such nature it is not sustainable? The last resort you have is revolution or civil war. Again, not the call of a foreign body to step in and start killing people.
Invading and starting a war which costs the live of innocent people is not the answer.
Like I said, voting for or wanting a separation does not guarantee you get what you desire.
It’s up to a country to determine how and if secession is possible. If the people of the complete country disagree with this separation the it will not happen and should not happen. Are the rest of a country any less of a factor? It is their country after all.
Discussing other situations specifically is tricky here. The formation of the US for example is incredibly difficult. Where did it start? The French, British or the colonist who formed the current country?
In the case we are discussing we have to deal with country as-is, the Ukraine as a whole. If secession is wanted then this region has to follow the rules and possibilities of Ukraine. iI’m not privy to these tbh.
What is not acceptable is invading that country and start killing people. Masquerading an election as valid while invading that country is not an option to consider as fair or legitimate.