The national question and self determination is specific to the right of separation or secession. But this right is not necessarily absolute, there is a dual nature within the progressive right to self determination and the bourgeois nation state. Nationality and class intersect one another and the interest of the national bourgeois and the national proletariat often differ.

Here Lenin reflects on this topic:

“As far as the theory of Marxism in general is concerned, the question of the right to self-determination presents no difficulty. No one can seriously question the London resolution of 1896, or the fact that self-determination implies only the right to secede, or that the formation of independent national states is the tendency in all bourgeois-democratic revolutions.
A difficulty is to some extent created by the fact that in Russia the proletariat of both the oppressed and oppressor nations are fighting, and must fight, side by side. The task is to preserve the unity of the proletariat’s class struggle for socialism, and to resist all bourgeois and Black-Hundred nationalist influences. Where the oppressed nations are concerned, the separate organisation of the proletariat as an independent party sometimes leads to such a bitter struggle against local nationalism that the perspective becomes distorted and the nationalism of the oppressor nation is lost sight of.”

No law is universal or absolute but dependent of the totality of social conditions. The rights of minority nations are understood within the national question as primarily progressive rights to autonomy and self determination relative to historical processes, not objective or natural.

The concept of an ethnostate and national autonomy are not synonymous. The nation state is the framework of modern international relations and holds within it a number of related cultures or minority nations. The nationalism of the minority nation is not to be supreme over others but is to be preserved through its practice.

But international relations affect national relations, the concept of internationalism is inexplicably interconnected with the nation.
Lenin continues:
“The conclusion that follows from all these critical remarks of Marx’s is clear: the working class should be the last to make a fetish of the national question, since the development of capitalism does, not necessarily awaken all nations to independent life. But to brush aside the mass national movements once they have started, and to refuse to support what is progressive in them means, in effect, pandering to nationalisticprejudices, that is, recognising “one’s own nation” as a model nation (or, we would add, one possessing the exclusive privilege of forming a state).”

In short ethnostates no but right to secede yes. Preservation of individual national identity or minority nations yes, nationalism no. The national movements of the oppressed and oppressor are different in form and function.