Well, there are about 300,000 Poles in the west of Belarus, a nation ruled by "Jewish neo-communists" and also probably Moskals. Encouraging a breakaway or joining of Poland there would be pretty much an act of war.
This is a different case. Belarus is an old nation-state with a titular nation and a lot of history while Moldova and Transnistria are just artificial Soviet creations where three different nationalities are lumped together.
Commies suck, but those under them should work to bring them down in the region where they are ruled by them and not just breakaway and leave the problem to someone else.
This is true in cases like Belarus: the relations between Poles and Belarussians are pretty good as far as I know so the Polish minority and Belarussians have common ground to cooperate against Jewkashenko who mistreats both of them. In Moldova and Transnistria this is not the case: Ukrainians will follow Ukrainian interests, Romanians Romanian interests and Russians Muscovian interests – there is simply no common ground for cooperation.
And what would be so bad about a Ukrainian minority in Romania? The Romanians hate the Soviet legacy as much as anyone and are logical allies for Ukraine and pretty much all Slavs if a greater Slavic coalition is ever to be formed.
Romanians treat us with condescension or even hate us and claim large parts of our land. I do not want my fellow Ukrainians to fall under their rule as much as I do not want them to fall under Muscovian rule.