That's curious indeed – but yes, all the official sources point to below 400m, and they will have been by geode triangulation, rather than phone GPS (which is not very accurate on vertical measurements). Equally, your phone may just be gauging elevation by interpolating nearby map data, and thus if AllTrails shows 438 it could even be based on that data (I see in the history that the ele=* tag has previously been 438, but was updated 4 years ago based on a reputable source, so AllTrails and/or your phone may be basing the altitude on old OSM data).

Additionally, I note that it includes a tag ele:wgs84=438.2 which suggests a difference by a different mercator, but I don't know enough about this to know why it should be higher than all other figures.

For now, I'd suggest the official sources are the correct ones; even the old British War Office maps show a blurry figure 12xx (feet) and no contours above 1250, suggesting the xx is between 50 and 99.