https://www.gamecity.ne.jp/nioh/update.html has the patch notes, releasing February 24th. The Google Translation of the Japanese isn't the best, but it seems to say that the 320 level was unintended and will be fixed, with existing 320 items retroactively de-leveled by some amount. It does not seem to say that 150 is the hard cap, just that 320 wasn't correct.
Can a native Japanese speaker help us understand the coming patch?
Here's what Google Translate spat out:
In "Forging" of "Blacksmith", with regard to the case where very high level equipment can be obtained rarely by satisfying certain conditions, it is a fault and equipment of a level greatly exceeding the assumption of the game balance emerges I will fix it because it will do. Even after Ver 1.04 is applied, high-level items will appear with low probability as before before application, but the level of appearance is corrected. Although we are very sorry, we will make a downward revision of the corresponding equipment at Ver 1.04, even for customers who already obtained the corresponding accessories. (Since it is a modification according to the original level, the higher the original level, the higher the level after correction)
I'm sure this will be a contentious topic, but I'd also like to point out a ton of bugs (and looks like some of the amrita exploits) are getting fixed too. Have to say, KT are on the ball with these turnaround times!