What kind of lettuce does it? Romaine, red leaf, iceberg, etc? I could see if romaine gave you indigestion because it's extremely fibrous and full of nutrients, but I'd be stunned if iceberg lettuce gave you issues since it's pretty much just water.
Kale and spinach are cruciferous vegetables (crucifer family) like broccoli, cabbage, mustard, rapini, bok choy and the like, so it's in a different family than lettuce even though they look similar AFAIK. Maybe I'm wrong, but it would explain why kale and spinach are a non-issue as they're related to each other but not lettuce. Don't let the "leafy greens" thing fool you.