Is astronomical twilight always seen when the sun is 18 degrees under the horizon or are there other factors that may influence it?
These values are convenient definitions rather than descriptions of an observable condition.
So astronomical twilight is defined to be when the sun is between 12 and 18 degrees below the horizon. Nothing special happens as the sun moves from 11.999 degrees to 12 degrees, and nothing special happens when the sun moves from 17.999 to 18 degrees.
So astronomical twilight isn't something that is seen. It's not an observable. But it is generally the case that by the time that the sun is 18 degrees below the horizon, the sky is pretty nearly as dark as it is going to get.
Clearly after the sun sets, the sky gets gradually darker and darker. It is a continuous process. It is convenient for humans to split up this continuum into chunks, and it is convenient for humans to do so using the position of the sun. There is nothing deeper about 18 degrees.
It’s the very definition of astronomical twilight.
Civil twilight is when the sun is 6° below the horizon.
Nautical twilight is at 12°.
Astronomical twilight is at 18°.
These are also defined by activities. Civil twilight is the period following sunset when it’s still bright enough outside to practice most activities. Nautical twilight is when it’s dark enough to see at least a few stars, so that sailors can use them for dead reckoning. Finally, astronomical twilight is when the sky is at its darkest.