CrowsFeet

I'm new to my X-T4, but battling with something that happened to me. Taking a passport photo for a child, in the viewfinder, the image looked great. Snapped the photo. But when I 'play' to view it - the photo was extremely dark. I thought, what you see in the view finder - is what you'll get. I'm mistaken I guess. Is this normal for a novice? I fixed it by going fully auto - as I was unable to work out why that happened. And Auto gave me good exposure.