I've managed to locate the exact place from the screenshot (there was enough identifying info for an overpass query so you might want to consider improving opsec if it's a privacy concern).

I think the reason why walking prefers to go the long way around is because the path parallel to the secondary road is marked as highway=footway, and walking algorithms generally prefer those over other types of paths. It is assumed that highway=footway is tended to and therefore more pleasant/fast to walk on compared to a general highway=path, which is just something that is maintained naturally because of people walking there. I guess surface=mud on the shorter path might also play into it - routers will generally penalize worse surfaces and instead suggest you to walk on firmer ones.

If that shorter path is actually "official" in some way and is pleasant to walk on, consider changing it to highway=footway, otherwise the router is probably behaving correctly by not sending you down a muddy shortcut.