Why is an image exported from GIMP much, much darker than it should be?

I was going to write a bug report for this on the official bug tracker, but their system is so fucking terrible I literally can't sign up despite them having two entire separate pages for creating accounts. One just silently fails, and the other accuses me of being malicious or using a VPN. So I guess they don't really want people contributing.

Here's what my image looks like in GIMP:

And here it is, after exporting as a JPG:

With these settings: