is there a name for the phenomenon where a user immediately assumes the smallest and lowest contrast button on an interface is the option they want, before actually reading any of the words?

This reversed cta thing is what I've been evolved to do. Always look for the opposite of a cta button and click it. This reconciles with the fact the incentives have been off for the past few years or decades.

    Enter through the narrow gate, for the gate is wide and the road is easy that leads to destruction, and there are many who take it. For the gate is narrow and the road is hard that leads to life, and there are few who find it.

    [Matthew 7:13-14]

I'm not aware of a specific term, another than just conditioning, but I am reminded of "banner blindness" - https://en.wikipedia.org/wiki/Banner_blindness

(I was definitely expecting a level to swap the contrast eventually as a trick.)

I was waiting for the cases where they inverted this. That would really trick me. But it didn't happen in the few cases I tried.

Being conditioned.