I have added an image of the full leaf as best I took. It looks like it may be an Amelanchier species leaf?

Saskatoon Serviceberry (Amelanchier alnifolia) is the only serviceberry species in British Colombia.

Similar individuals are noted on Prunus. Whatever this is, it does indeed bear a striking resemblance to Pristiphora cincta, but that species is only confirmed on Betula and Vaccinium, and differs in having the yellow blotches distinctly divided.

In 1895, Dyar included in his key to sawfly larvae an unidentified species, which he found feeding on the edges of both Amelanchier and Prunus leaves in New Hampshire, described as "vinous [the color of red wine] with obliquely cut white fat patches." This sure seems to be what he was describing, and I guess that's what you're referring to. Although Dyar's paper only mentions Prunus as a host, his notebook also indicated he found these larvae on Amelanchier canadensis (see Smith 1986).

Yes, I suspect I know exactly what species this is, and will hopefully be publishing that information soon. I've seen several other records that would support Amelanchier (plus one which doesn't seem to track with any likely host, so probably the host range is somewhat flexible).

Looking forward to that publication! Here's one that's said to be on hawthorn: https://www.inaturalist.org/observations/79308099
I get the sense from leafminer and sawfly host ranges that some Rosaceae genera aren't very chemically differentiated. Not to mention the existence of intergeneric hybrids like Sorbaronia.