The feedback in the Evolver works in the following way:
We'll start with the description from the manual.
Feedback 1: 0…100 – Sets the amount of feedback from the summed output of all the delay taps to the input of the delay.
Feedback 2: 0…100 – Sets the amount of feedback from the summed output of all the delay taps to the input of the filter for more extreme effects.
Because Feedback 2 loops back to the input of the filter, the effect can be pretty subtle, especially if the cutoff and filter/amp envelopes are such that the repeats are diminished in volume and brightness as they loop back through. On a lot of patches, when you turn up Feedback 2, it sounds like you're only hearing the initial repeat, but if you futz around with the filter and envelope settings, you can hear subsequent repeats (though maybe not obviously).
Also, since Feedback 2 goes through the filter/amp, if the filter or amp is off, no Feedback 2!
Feedback 1 is local to the delay, so it will do repeats etc. Feedback 2 is more like the Main Feedback loop (and less like Feedback 1), in that it only works with sustained sounds, and in many cases only fills in the sound subtly. Short delays will scream like Feedback. With long delays on sustained sounds, you don’t hear the repeat quite as much.
It can also be more obvious if you have more than 1 of the delay taps being used.