I’m guessing the mapper who changed it to ‘trunk’ did so to make the tagging consistent with the highway on the Maine side of the border.
My opinion on this is the same as before, when asked about the tagging of Manitoba PTH 10 from Brandon to the border: Trunk Road connection between Manitoba and Central North Dakota - #2 by hoserab
Anecdotally Route 138 is waaaaaaaaay more important in Québec than Route 173 to Maine.