It'd stay belonging to New York. NY has no reason to sell it to Pennsylvania, especially in this TL where they're going to be rival nations rather than friends.
The Erie Triangle was never part of NY, at least not by the time such an entity as 'the Erie Triangle' came to exist as a territorial issue. NY state settled the border with PA in 1774 by agreeing to draw a line along the 42 parallel from the Delaware river west to Lake Erie. Conceivably this would give NY 'the Erie Triangle' except that land north of the border was also claimed by MA under its sea to sea charter. NY settled its dispute with Mass over Western NY in 1787 by the Treaty of Hartford by giving the State of MA right of preemptive purchase (from Iroquois for next to nothing) and sale (to settlers for $$$) of lands west of the preemption line (just west of modern day Geneva NY). But the land itself would become part of the state of NY. MA got the cash by selling the land to settlers and NY got to keep it long term. This would have theoretically left the triangle in NY except for the NY's cession of western lands.
In the interim between those two land and border settlements NY agreed to the Cession Act of 1780 which ceded claims to all Indian land in the North West. It was hoped that if they gave up claims there the Federal government and other states would look favorably upon their claims to the area that became Vermont (which they didn't). The act was approved by Congress in 1782. The post 1782 western border of NY state was a meridian (N-S line) drawn south from the western bend (edge/shore) of Lake Ontario.
At the time it was assumed that Lake Ontario actually extended much further west than it did so it was assumed the Pennsylvania border along the 42nd parallel would not cross that meridian until well into Lake Erie. Thus NY state assumed it was only giving away land further west in Michigan and the Ohio Valley by agreeing to the Cession Act. When a survey was finally undertaken in '86-'87 (the British were reluctant to let American surveyors into Canada) and a measurement was taken of what longitude Lake Ontario ended at it was found to be east of Presque Isle. So the 'Erie Triangle' came into existence as land that both NY and Mass had given up claim to, the former under false assumptions of local geography, and to which PA had no claim under the border it negotiated with NY.
By default the Triangle became Federal land same as the other territories of the Northwest ceded by NY (1782), VA (1784), MA (1787) and CT (1786 but excepting the Reserve along Lake Erie). But it was far too small to ever be made its own state like OH, Indiana etc etc and completely surrounded by NY and PA. So NY and PA then submitted competing bids to buy the land from the Federal government. PA had the better lobbyist and won and was able to purchase the rights to the land from the Feds, after which they then purchased the land itself from the Iroquois.
Back to the original question. I assume by Articles failing - disintegration of the US you mean the states go their separate ways after some period of trying and failing to make it work under the Articles, so sometime between 1781 and 1789. So a lot depends on exactly when and how the Federal government under the articles breaks down. The Triangle is under NYS claim until 1780 (submitted by NY) or 1782 (ratified by Congress) and under MA claim until the 1787 Treaty of Hartford. NY probably still cedes the terr since they did it so early on, the US would have to fall apart just as the Articles go into force to change this. I assume MA still agrees to the Treaty because they got a good deal with Preemption since all they were really after was money. Two sovereign states would likely still negotiate such a deal. So its really a matter of how federal lands are split up when the states go their separate ways since no state is likely to have legal claim to the land by that point.
Unlike Virginia and CT New York state had no 'reserve' in the Western territories. It gave up all claim to such lands without condition in 1780 and instead settled its veterans in CNY around modern Syracuse and Utica (which is why all towns in the region are named for Roman generals; Pompey, Manlius, Camillus, Marcellus etc etc). So it has no immediate financial imperative to recover western lands and since it already has a generous share of Lake Erie's southern and eastern shore it has no geo-political imperative. NY is still likely to cry foul and say they never meant to cede claim to the triangle but its hard to say how hard they'd be willing to fight it if came to that.
Another possibility is that the US falls apart over the issue of ceding western land. There was a hold up on Virginia's cession because they included clauses to honor previous land grants which is why it took four years to be approved. So if the US falls apart before NY cedes its western lands it still has claim to the triangle (and much more to the west) and probably will retain it in any settlement to the detriment of PA. PA after all never had any claim to the triangle or to the Ohio valley so pressing claims to western land can't be based on law so much as economic or geo-politcal need which NY, CT, VA et all are not likely to care too much about.
Sorry this is so long but as I'm originally from WNY this is an issue I not only know a little about but have a personal connection to, so to speak.