Supply routing has a few bugs

[STABBY5]

If you don't try and influence where supply travels, it is fine. Playing as Japan I have noticed a few things that aren't working as they should.

Blocking sea zones will not allow supply and trade to reroute. I built railways from Vietnam to Korea to minimize my vulnerable convoys. The game still preferred to use the sea so I first avoided the sea zone which will not make it use the land route as it should. Then I blocked it and instead of rerouting, it cut off the resources. If I reload however, it will correctly route it over land to Korea and then by ship to Japan. I cannot touch it, or it will break and I'll have to reload. In a similar scenario I cannot trade with the soviet union via Siberia. It will only come from Europe or I get nothing.

Sea routes are heavily preferred. This causes silly things like after the fall of France, Germanys coastal garrisons are supplied via the sea even with a perfectly valid land route. You have to forbit the sea zone or you will not only bleed convoys but have no supply because the chosen route has 0% efficiency. Unless you micro this silliness you will sit in your own land attritioning to death.

 

[Andrew0Red]

In the real world, shipping has always been cheaper than overland. Even today, it still is.

And yes, it's very weird that HoI4 chooses the realistic path....

 

[Kanitatlan]

In a similar scenario I cannot trade with the soviet union via Siberia. It will only come from Europe or I get nothing.

This is a well established issue and is actually even worse than you think. If you invade Russia as Japan it is important to avoid taking a port in Europe as long as possible so that you can keep everything operating via Vladivostok. I haven't checked how supply works in this situation since NSB came it but the trade arrangements for resources from European conquests (including European Russia) insist on using the available European port even if the sea route is blocked and, for example, capturing Archangelsk can suddenly cut off the supply of all those essential resources.

In general, the affect of current trade and supply arrangements is far more serious for Japan than it is for any other nation. Just think, move you capital onto the continent and suddenly all of your trade is no issue, but keep it in Japan and you must uses ships sailing from European ports. A number of mods put a straights connection between the Kuriles and Kamchatka to specifically address this issue.