Right.
That will be because the banana is terrible choice for some of us and only OK for others. Rare to find anyone where they are wonderful. Most people have trouble with them, frankly. I can only do half a banana, and that's a bit risky. Patti can't do any banana. LOL
Very fast-acting so you had what, 30g in the sandwich, maybe more depending on bread and filling and ? 20g or more in the banana. So at least 50g, all fast-acting SPLAT it lands all together, simultaneously, at the same time too, in your blood. Nothing much to slow it down. The butter on the bread isn't enough to save you from that amount of carb. Unless the sandwich also contained half a pound of cheddar?
50g fast acting carb = a rise of 15.0 on my meter. OK some of the insulin will get there by 2 hours, but depends on how long it lasts in you. Usually between 4 and 5 hours, but it tails off in the last hour-ish unless it's Humalog, which sometimes has a sting in it's tail. Probably gonna do most of what it does in the first 3 hours.
Try not eating a whole one, or eating a whole one and not the sandwich, or something. And test test test !