> The international-law disputes over who signed what treaty, 3 states ago, seem moot and trivial compared with the principle of self-determination.
Here lies the flaw in your thinking. An obvious issue is that Transnistrian secession would impinge on Moldovan sovereignty. This is similar to how Catalonia is not allowed to secede from Spain. Some counterexamples are Kosovo seceding from Serbia (supported by USA and UN because of Serbia's terrible international standing at that moment, specifically because of the ethnic cleansing employed by the Serbian government and army in the war) and Algeria seceding from France (which the French agreed to to end the Algerian War). So I think the reason for Transnistria not being recognized is that Moldova is in relatively good international standing (at least compared to Serbia around the time of the Kosovo War), thus no internationally recognized country wants to attack Moldova's sovereignty, for kind of the same reasons that most people try not to break any laws. (And those that would be willing to "break the law" presumably have nothing to gain from it.)
Also see https://en.wikipedia.org/wiki/Self-determination
> The principle does not state how the decision is to be made, nor what the outcome should be, whether it be independence, federation, protection, some form of autonomy or full assimilation. Neither does it state what the delimitation between peoples should be—nor what constitutes a people. There are conflicting definitions and legal criteria for determining which groups may legitimately claim the right to self-determination.