Acorn TV, the upstart streaming service specializing in British television, is still a tiny operation, with about 115,000 paid subscribers. Nonetheless, its fast growth is causing outsized concern at PBS and Masterpiece, public television’s longstanding home for British drama. Brewing tensions came to a head over rights to the final three episodes in David Suchet’s marathon 70-program portrayal of Agatha Christie’s Belgian detective Hercule Poirot. As a result of the rift, Acorn TV premiered the episodes to its streaming subscribers in August and syndicated them directly to local public TV stations, with Masterpiece nowhere in the picture. The broadcast window for the finale’s broadcast opens Nov.