Did your amplifier produce hissing sound when there is no cable plugged in? If nothing connected, then it can be grounding problem.

If the hissing sound only become loud when you plug in your guitar, it can be the wire problem.

The power supply noise usually produces humming sound.

I have no idea what kind of pick-up you use for your acoustic guitar. Built-in or separate purchase. See this article if it helps

https://joneruizguitar.com/guitar-amp-hissi...-you-must-know/

Since both guitar and amplifier are Yamaha, I think it is not about cheap amplifier.

The signal from guitar is very small and the amplifier does have high gain to pick-up the signal. The cable itself can pick up signal from surrounding. Maybe there are other devices nearby that produce noise. That is why you need to avoid long cable. If you still need to have long cable, then try to use pre-amp. This way the cable from guitar can be short. The signal from pre-amp to main amp is no longer weak signal. It is amplified high enough to be above ground noise level.