I am shocked, shocked that countries may develop inconsistent policies as they react to separatism in other countries. Ok, I am not. After all, my quantitative piece on this stuff is entitled: Discrimination in International Relations. That and the related book, The Ties That Divide, argue that countries will support those secessionists that they "like" and oppose those that they "dislike." What do I mean by like? Domestic politics FTW! That is, often but not always, domestic audiences in country x will have a preference about what happens in country y based on whether the folks in x have ties to anyone in y, particularly ethnic ties. When ethnic ties do not exist, other stuff may matter more, such as precedents (I am a precedent-skeptic), norms (I am a norm-skeptic), or strategic interests (yeah, those make sense).
So, to take a local and obvious example, Canada is taking a strong position on this. Some would say this is because Canada is vulnerable to secession. To that, I say booshwah. Canada recognized Kosovo. Indeed, one of the key targets of Ties That Divide was the conventional wisdom that vulnerability to secession deters support for secession elsewhere. Anyhow, Canada might be doing this because it is a good friend of countries that are sort of friend with NATO. But part of it is that Ukrainian-Canadians are a key constituency in Canada.
Sure, it is not just about ethnic ties, but countries react to secessionist movements based on the context of each one, rather than to any over-arching principle. Russia supports secessionists it likes (Abkhaz, South Osettian, Transnistrian, Crimean Russian) and not those it dislikes (Kosovo). The US tends not to like secession because it disrupts the status quo, and as the dominant power in the system, the US likes things as it is. But when repression of an ethnic group is a greater threat to regional stability (Kosovo) or when movie stars keep pushing (South Sudan?), the US will support secession.
There is heaps of good normative work by Allen Buchanan, Margaret Moore, Jason Sorens and others on the oughts of secession and recognition. I have my own views, partly guided by these folks and partly shaped by my prejudices (as a Yankee) and as a scholar of separatism. I tend not to support secession in democracies since there are other ways to get one's interests represented. Quebec and Scotland, for instance, can get much from their current governments and are not repressed. But when facing a government that repeatedly targeted ethnic minorities and revoked whatever autonomy, then, yeah, secession makes sense (Kosovo, South Sudan, East Timor).
Anyhow, the point here is that in International Relations, one should expect inconsistency in irredentism and in secession. Otherwise, one is a foolish hobgoblin or something like that.