Note: Using this exploit might affect game balance, so you have been warned.
Having played this game for a few days on the hard level, it's become painfully evident that there's a cashflow problem with the game. Here's how you can alleviate some of it through simple diplomacy.
This trick will only work on factions which directly border your territory. All you have to do is go to the diplomacy screen, pick a bordering faction and offer them Military Access. Always start off with 5 turns. The probably for success should be listed as 'High'. Then demand a cash payment from them. The amount they will agree to should be between 750 to 1750 koku. You will know when you hit their payout limit when the success meter changes to 'Moderate'.
You can then either wait 5 turns until the agreement expires and farm them for more money or if you're really strapped for cash, you can immediately offer them 10 turns of Military Access and milk them for more money in the same turn (though you will get less cash this way). That being said, you can incrementally move up to 20 turns and unlimited on the same turn as well.
You can get on average around 1000 koku from each neighbor every 5 turns. I personally find that even this big boost in income barely covers expenses, but it's definitely better than nothing. If there is one caveat, it's that you will get flagged as 'Dishonoring Treaties' among other things if you attack a faction while the agreement is in force so you will have to plan your offensives in advance.
Maybe our daimyos should just quit their day jobs and be diplomatic gold farmers instead.
I don't think this qualifies as a bug, but it's certainly an exploit.
Have fun and don't let this trick ruin your game.